The tracing ring-buffers can be stored on disk or sent to network
without any copy via splice. However the later doesn't allow real time
processing of the traces. A solution is to give userspace direct access
to the ring-buffer pages via a mapping. An application can now become a
consumer of the ring-buffer, in a similar fashion to what trace_pipe
offers.
Support for this new feature can already be found in libtracefs from
version 1.8, when built with EXTRA_CFLAGS=-DFORCE_MMAP_ENABLE.
Vincent
v20 -> v21:
* Collect Ack
* Add .gitignore
* Few nits
* Remove meta-page padding (zero-page not supported by vm_insert_pages)
* Remove single-usage macros
* Move vma flags handling into ring-buffer.c
v19 -> v20:
* Fix typos in documentation.
* Remove useless mmap open and fault callbacks.
* add mm.h include for vm_insert_pages
v18 -> v19:
* Use VM_PFNMAP and vm_insert_pages
* Allocate ring-buffer subbufs with __GFP_COMP
* Pad the meta-page with the zero-page to align on the subbuf_order
* Extend the ring-buffer test with mmap() dedicated suite
v17 -> v18:
* Fix lockdep_assert_held
* Fix spin_lock_init typo
* Fix CONFIG_TRACER_MAX_TRACE typo
v16 -> v17:
* Documentation and comments improvements.
* Create get/put_snapshot_map() for clearer code.
* Replace kzalloc with kcalloc.
* Fix -ENOMEM handling in rb_alloc_meta_page().
* Move flush(cpu_buffer->reader_page) behind the reader lock.
* Move all inc/dec of cpu_buffer->mapped behind reader lock and buffer
mutex. (removes READ_ONCE/WRITE_ONCE accesses).
v15 -> v16:
* Add comment for the dcache flush.
* Remove now unnecessary WRITE_ONCE for the meta-page.
v14 -> v15:
* Add meta-page and reader-page flush. Intends to fix the mapping
for VIVT and aliasing-VIPT data caches.
* -EPERM on VM_EXEC.
* Fix build warning !CONFIG_TRACER_MAX_TRACE.
v13 -> v14:
* All cpu_buffer->mapped readers use READ_ONCE (except for swap_cpu)
* on unmap, sync meta-page teardown with the reader_lock instead of
the synchronize_rcu.
* Add a dedicated spinlock for trace_array ->snapshot and ->mapped.
(intends to fix a lockdep issue)
* Add kerneldoc for flags and Reserved fields.
* Add kselftest for snapshot/map mutual exclusion.
v12 -> v13:
* Swap subbufs_{touched,lost} for Reserved fields.
* Add a flag field in the meta-page.
* Fix CONFIG_TRACER_MAX_TRACE.
* Rebase on top of trace/urgent.
* Add a comment for try_unregister_trigger()
v11 -> v12:
* Fix code sample mmap bug.
* Add logging in sample code.
* Reset tracer in selftest.
* Add a refcount for the snapshot users.
* Prevent mapping when there are snapshot users and vice versa.
* Refine the meta-page.
* Fix types in the meta-page.
* Collect Reviewed-by.
v10 -> v11:
* Add Documentation and code sample.
* Add a selftest.
* Move all the update to the meta-page into a single
rb_update_meta_page().
* rb_update_meta_page() is now called from
ring_buffer_map_get_reader() to fix NOBLOCK callers.
* kerneldoc for struct trace_meta_page.
* Add a patch to zero all the ring-buffer allocations.
v9 -> v10:
* Refactor rb_update_meta_page()
* In-loop declaration for foreach_subbuf_page()
* Check for cpu_buffer->mapped overflow
v8 -> v9:
* Fix the unlock path in ring_buffer_map()
* Fix cpu_buffer cast with rb_work_rq->is_cpu_buffer
* Rebase on linux-trace/for-next (3cb3091138ca0921c4569bcf7ffa062519639b6a)
v7 -> v8:
* Drop the subbufs renaming into bpages
* Use subbuf as a name when relevant
v6 -> v7:
* Rebase onto lore.kernel.org/lkml/[email protected]/
* Support for subbufs
* Rename subbufs into bpages
v5 -> v6:
* Rebase on next-20230802.
* (unsigned long) -> (void *) cast for virt_to_page().
* Add a wait for the GET_READER_PAGE ioctl.
* Move writer fields update (overrun/pages_lost/entries/pages_touched)
in the irq_work.
* Rearrange id in struct buffer_page.
* Rearrange the meta-page.
* ring_buffer_meta_page -> trace_buffer_meta_page.
* Add meta_struct_len into the meta-page.
v4 -> v5:
* Trivial rebase onto 6.5-rc3 (previously 6.4-rc3)
v3 -> v4:
* Add to the meta-page:
- pages_lost / pages_read (allow to compute how full is the
ring-buffer)
- read (allow to compute how many entries can be read)
- A reader_page struct.
* Rename ring_buffer_meta_header -> ring_buffer_meta
* Rename ring_buffer_get_reader_page -> ring_buffer_map_get_reader_page
* Properly consume events on ring_buffer_map_get_reader_page() with
rb_advance_reader().
v2 -> v3:
* Remove data page list (for non-consuming read)
** Implies removing order > 0 meta-page
* Add a new meta page field ->read
* Rename ring_buffer_meta_page_header into ring_buffer_meta_header
v1 -> v2:
* Hide data_pages from the userspace struct
* Fix META_PAGE_MAX_PAGES
* Support for order > 0 meta-page
* Add missing page->mapping.
Vincent Donnefort (5):
ring-buffer: Allocate sub-buffers with __GFP_COMP
ring-buffer: Introducing ring-buffer mapping functions
tracing: Allow user-space mapping of the ring-buffer
Documentation: tracing: Add ring-buffer mapping
ring-buffer/selftest: Add ring-buffer mapping test
Documentation/trace/index.rst | 1 +
Documentation/trace/ring-buffer-map.rst | 106 +++++
include/linux/ring_buffer.h | 6 +
include/uapi/linux/trace_mmap.h | 48 ++
kernel/trace/ring_buffer.c | 412 +++++++++++++++++-
kernel/trace/trace.c | 104 ++++-
kernel/trace/trace.h | 1 +
.../testing/selftests/ring-buffer/.gitignore | 1 +
tools/testing/selftests/ring-buffer/Makefile | 8 +
tools/testing/selftests/ring-buffer/config | 2 +
.../testing/selftests/ring-buffer/map_test.c | 288 ++++++++++++
11 files changed, 966 insertions(+), 11 deletions(-)
create mode 100644 Documentation/trace/ring-buffer-map.rst
create mode 100644 include/uapi/linux/trace_mmap.h
create mode 100644 tools/testing/selftests/ring-buffer/.gitignore
create mode 100644 tools/testing/selftests/ring-buffer/Makefile
create mode 100644 tools/testing/selftests/ring-buffer/config
create mode 100644 tools/testing/selftests/ring-buffer/map_test.c
base-commit: 7604256cecef34a82333d9f78262d3180f4eb525
--
2.44.0.769.g3c40516874-goog
In preparation for the ring-buffer memory mapping, allocate compound
pages for the ring-buffer sub-buffers to enable us to map them to
user-space with vm_insert_pages().
Signed-off-by: Vincent Donnefort <[email protected]>
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 25476ead681b..cc9ebe593571 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1524,7 +1524,7 @@ static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
list_add(&bpage->list, pages);
page = alloc_pages_node(cpu_to_node(cpu_buffer->cpu),
- mflags | __GFP_ZERO,
+ mflags | __GFP_COMP | __GFP_ZERO,
cpu_buffer->buffer->subbuf_order);
if (!page)
goto free_pages;
@@ -1609,7 +1609,7 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
cpu_buffer->reader_page = bpage;
- page = alloc_pages_node(cpu_to_node(cpu), GFP_KERNEL | __GFP_ZERO,
+ page = alloc_pages_node(cpu_to_node(cpu), GFP_KERNEL | __GFP_COMP | __GFP_ZERO,
cpu_buffer->buffer->subbuf_order);
if (!page)
goto fail_free_reader;
@@ -5579,7 +5579,7 @@ ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu)
goto out;
page = alloc_pages_node(cpu_to_node(cpu),
- GFP_KERNEL | __GFP_NORETRY | __GFP_ZERO,
+ GFP_KERNEL | __GFP_NORETRY | __GFP_COMP | __GFP_ZERO,
cpu_buffer->buffer->subbuf_order);
if (!page) {
kfree(bpage);
--
2.44.0.769.g3c40516874-goog
In preparation for allowing the user-space to map a ring-buffer, add
a set of mapping functions:
ring_buffer_{map,unmap}()
And controls on the ring-buffer:
ring_buffer_map_get_reader() /* swap reader and head */
Mapping the ring-buffer also involves:
A unique ID for each subbuf of the ring-buffer, currently they are
only identified through their in-kernel VA.
A meta-page, where are stored ring-buffer statistics and a
description for the current reader
The linear mapping exposes the meta-page, and each subbuf of the
ring-buffer, ordered following their unique ID, assigned during the
first mapping.
Once mapped, no subbuf can get in or out of the ring-buffer: the buffer
size will remain unmodified and the splice enabling functions will in
reality simply memcpy the data instead of swapping subbufs.
CC: <[email protected]>
Signed-off-by: Vincent Donnefort <[email protected]>
diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index dc5ae4e96aee..96d2140b471e 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -6,6 +6,8 @@
#include <linux/seq_file.h>
#include <linux/poll.h>
+#include <uapi/linux/trace_mmap.h>
+
struct trace_buffer;
struct ring_buffer_iter;
@@ -223,4 +225,8 @@ int trace_rb_cpu_prepare(unsigned int cpu, struct hlist_node *node);
#define trace_rb_cpu_prepare NULL
#endif
+int ring_buffer_map(struct trace_buffer *buffer, int cpu,
+ struct vm_area_struct *vma);
+int ring_buffer_unmap(struct trace_buffer *buffer, int cpu);
+int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu);
#endif /* _LINUX_RING_BUFFER_H */
diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
new file mode 100644
index 000000000000..b682e9925539
--- /dev/null
+++ b/include/uapi/linux/trace_mmap.h
@@ -0,0 +1,46 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _TRACE_MMAP_H_
+#define _TRACE_MMAP_H_
+
+#include <linux/types.h>
+
+/**
+ * struct trace_buffer_meta - Ring-buffer Meta-page description
+ * @meta_page_size: Size of this meta-page.
+ * @meta_struct_len: Size of this structure.
+ * @subbuf_size: Size of each sub-buffer.
+ * @nr_subbufs: Number of subbfs in the ring-buffer, including the reader.
+ * @reader.lost_events: Number of events lost at the time of the reader swap.
+ * @reader.id: subbuf ID of the current reader. ID range [0 : @nr_subbufs - 1]
+ * @reader.read: Number of bytes read on the reader subbuf.
+ * @flags: Placeholder for now, 0 until new features are supported.
+ * @entries: Number of entries in the ring-buffer.
+ * @overrun: Number of entries lost in the ring-buffer.
+ * @read: Number of entries that have been read.
+ * @Reserved1: Internal use only.
+ * @Reserved2: Internal use only.
+ */
+struct trace_buffer_meta {
+ __u32 meta_page_size;
+ __u32 meta_struct_len;
+
+ __u32 subbuf_size;
+ __u32 nr_subbufs;
+
+ struct {
+ __u64 lost_events;
+ __u32 id;
+ __u32 read;
+ } reader;
+
+ __u64 flags;
+
+ __u64 entries;
+ __u64 overrun;
+ __u64 read;
+
+ __u64 Reserved1;
+ __u64 Reserved2;
+};
+
+#endif /* _TRACE_MMAP_H_ */
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index cc9ebe593571..84f8744fa110 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -9,6 +9,7 @@
#include <linux/ring_buffer.h>
#include <linux/trace_clock.h>
#include <linux/sched/clock.h>
+#include <linux/cacheflush.h>
#include <linux/trace_seq.h>
#include <linux/spinlock.h>
#include <linux/irq_work.h>
@@ -26,6 +27,7 @@
#include <linux/list.h>
#include <linux/cpu.h>
#include <linux/oom.h>
+#include <linux/mm.h>
#include <asm/local64.h>
#include <asm/local.h>
@@ -338,6 +340,7 @@ struct buffer_page {
local_t entries; /* entries on this page */
unsigned long real_end; /* real end of data */
unsigned order; /* order of the page */
+ u32 id; /* ID for external mapping */
struct buffer_data_page *page; /* Actual data page */
};
@@ -484,6 +487,12 @@ struct ring_buffer_per_cpu {
u64 read_stamp;
/* pages removed since last reset */
unsigned long pages_removed;
+
+ unsigned int mapped;
+ struct mutex mapping_lock;
+ unsigned long *subbuf_ids; /* ID to subbuf VA */
+ struct trace_buffer_meta *meta_page;
+
/* ring buffer pages to update, > 0 to add, < 0 to remove */
long nr_pages_to_update;
struct list_head new_pages; /* new pages to add */
@@ -1599,6 +1608,7 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);
init_waitqueue_head(&cpu_buffer->irq_work.waiters);
init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
+ mutex_init(&cpu_buffer->mapping_lock);
bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
GFP_KERNEL, cpu_to_node(cpu));
@@ -1789,8 +1799,6 @@ bool ring_buffer_time_stamp_abs(struct trace_buffer *buffer)
return buffer->time_stamp_abs;
}
-static void rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer);
-
static inline unsigned long rb_page_entries(struct buffer_page *bpage)
{
return local_read(&bpage->entries) & RB_WRITE_MASK;
@@ -5211,6 +5219,22 @@ static void rb_clear_buffer_page(struct buffer_page *page)
page->read = 0;
}
+static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
+{
+ struct trace_buffer_meta *meta = cpu_buffer->meta_page;
+
+ meta->reader.read = cpu_buffer->reader_page->read;
+ meta->reader.id = cpu_buffer->reader_page->id;
+ meta->reader.lost_events = cpu_buffer->lost_events;
+
+ meta->entries = local_read(&cpu_buffer->entries);
+ meta->overrun = local_read(&cpu_buffer->overrun);
+ meta->read = cpu_buffer->read;
+
+ /* Some archs do not have data cache coherency between kernel and user-space */
+ flush_dcache_folio(virt_to_folio(cpu_buffer->meta_page));
+}
+
static void
rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
{
@@ -5255,6 +5279,9 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->lost_events = 0;
cpu_buffer->last_overrun = 0;
+ if (cpu_buffer->mapped)
+ rb_update_meta_page(cpu_buffer);
+
rb_head_page_activate(cpu_buffer);
cpu_buffer->pages_removed = 0;
}
@@ -5469,6 +5496,12 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
cpu_buffer_a = buffer_a->buffers[cpu];
cpu_buffer_b = buffer_b->buffers[cpu];
+ /* It's up to the callers to not try to swap mapped buffers */
+ if (WARN_ON_ONCE(cpu_buffer_a->mapped || cpu_buffer_b->mapped)) {
+ ret = -EBUSY;
+ goto out;
+ }
+
/* At least make sure the two buffers are somewhat the same */
if (cpu_buffer_a->nr_pages != cpu_buffer_b->nr_pages)
goto out;
@@ -5733,7 +5766,8 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
* Otherwise, we can simply swap the page with the one passed in.
*/
if (read || (len < (commit - read)) ||
- cpu_buffer->reader_page == cpu_buffer->commit_page) {
+ cpu_buffer->reader_page == cpu_buffer->commit_page ||
+ cpu_buffer->mapped) {
struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
unsigned int rpos = read;
unsigned int pos = 0;
@@ -5956,6 +5990,11 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
cpu_buffer = buffer->buffers[cpu];
+ if (cpu_buffer->mapped) {
+ err = -EBUSY;
+ goto error;
+ }
+
/* Update the number of pages to match the new size */
nr_pages = old_size * buffer->buffers[cpu]->nr_pages;
nr_pages = DIV_ROUND_UP(nr_pages, buffer->subbuf_size);
@@ -6057,6 +6096,367 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
}
EXPORT_SYMBOL_GPL(ring_buffer_subbuf_order_set);
+static int rb_alloc_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
+{
+ struct page *page;
+
+ if (cpu_buffer->meta_page)
+ return 0;
+
+ page = alloc_page(GFP_USER | __GFP_ZERO);
+ if (!page)
+ return -ENOMEM;
+
+ cpu_buffer->meta_page = page_to_virt(page);
+
+ return 0;
+}
+
+static void rb_free_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
+{
+ unsigned long addr = (unsigned long)cpu_buffer->meta_page;
+
+ free_page(addr);
+ cpu_buffer->meta_page = NULL;
+}
+
+static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
+ unsigned long *subbuf_ids)
+{
+ struct trace_buffer_meta *meta = cpu_buffer->meta_page;
+ unsigned int nr_subbufs = cpu_buffer->nr_pages + 1;
+ struct buffer_page *first_subbuf, *subbuf;
+ int id = 0;
+
+ subbuf_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
+ cpu_buffer->reader_page->id = id++;
+
+ first_subbuf = subbuf = rb_set_head_page(cpu_buffer);
+ do {
+ if (WARN_ON(id >= nr_subbufs))
+ break;
+
+ subbuf_ids[id] = (unsigned long)subbuf->page;
+ subbuf->id = id;
+
+ rb_inc_page(&subbuf);
+ id++;
+ } while (subbuf != first_subbuf);
+
+ /* install subbuf ID to kern VA translation */
+ cpu_buffer->subbuf_ids = subbuf_ids;
+
+ meta->meta_page_size = PAGE_SIZE;
+ meta->meta_struct_len = sizeof(*meta);
+ meta->nr_subbufs = nr_subbufs;
+ meta->subbuf_size = cpu_buffer->buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
+
+ rb_update_meta_page(cpu_buffer);
+}
+
+static struct ring_buffer_per_cpu *
+rb_get_mapped_buffer(struct trace_buffer *buffer, int cpu)
+{
+ struct ring_buffer_per_cpu *cpu_buffer;
+
+ if (!cpumask_test_cpu(cpu, buffer->cpumask))
+ return ERR_PTR(-EINVAL);
+
+ cpu_buffer = buffer->buffers[cpu];
+
+ mutex_lock(&cpu_buffer->mapping_lock);
+
+ if (!cpu_buffer->mapped) {
+ mutex_unlock(&cpu_buffer->mapping_lock);
+ return ERR_PTR(-ENODEV);
+ }
+
+ return cpu_buffer;
+}
+
+static void rb_put_mapped_buffer(struct ring_buffer_per_cpu *cpu_buffer)
+{
+ mutex_unlock(&cpu_buffer->mapping_lock);
+}
+
+/*
+ * Fast-path for rb_buffer_(un)map(). Called whenever the meta-page doesn't need
+ * to be set-up or torn-down.
+ */
+static int __rb_inc_dec_mapped(struct ring_buffer_per_cpu *cpu_buffer,
+ bool inc)
+{
+ unsigned long flags;
+
+ lockdep_assert_held(&cpu_buffer->mapping_lock);
+
+ if (inc && cpu_buffer->mapped == UINT_MAX)
+ return -EBUSY;
+
+ if (WARN_ON(!inc && cpu_buffer->mapped == 0))
+ return -EINVAL;
+
+ mutex_lock(&cpu_buffer->buffer->mutex);
+ raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+
+ if (inc)
+ cpu_buffer->mapped++;
+ else
+ cpu_buffer->mapped--;
+
+ raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+ mutex_unlock(&cpu_buffer->buffer->mutex);
+
+ return 0;
+}
+
+/*
+ * +--------------+ pgoff == 0
+ * | meta page |
+ * +--------------+ pgoff == 1
+ * | subbuffer 0 |
+ * | |
+ * +--------------+ pgoff == (1 + (1 << subbuf_order))
+ * | subbuffer 1 |
+ * | |
+ * ...
+ */
+#ifdef CONFIG_MMU
+static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
+ struct vm_area_struct *vma)
+{
+ unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
+ unsigned int subbuf_pages, subbuf_order;
+ struct page **pages;
+ int p = 0, s = 0;
+ int err;
+
+ if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
+ !(vma->vm_flags & VM_MAYSHARE))
+ return -EPERM;
+
+ vm_flags_mod(vma,
+ VM_MIXEDMAP | VM_PFNMAP |
+ VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
+ VM_MAYWRITE);
+
+ lockdep_assert_held(&cpu_buffer->mapping_lock);
+
+ subbuf_order = cpu_buffer->buffer->subbuf_order;
+ subbuf_pages = 1 << subbuf_order;
+
+ nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
+ nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
+
+ vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+ if (!vma_pages || vma_pages > nr_pages)
+ return -EINVAL;
+
+ nr_pages = vma_pages;
+
+ pages = kcalloc(nr_pages, sizeof(*pages), GFP_KERNEL);
+ if (!pages)
+ return -ENOMEM;
+
+ if (!pgoff) {
+ pages[p++] = virt_to_page(cpu_buffer->meta_page);
+
+ /*
+ * TODO: Align sub-buffers on their size, once
+ * vm_insert_pages() supports the zero-page.
+ */
+ } else {
+ /* Skip the meta-page */
+ pgoff--;
+
+ if (pgoff % subbuf_pages) {
+ err = -EINVAL;
+ goto out;
+ }
+
+ s += pgoff / subbuf_pages;
+ }
+
+ while (s < nr_subbufs && p < nr_pages) {
+ struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);
+ int off = 0;
+
+ for (; off < (1 << (subbuf_order)); off++, page++) {
+ if (p >= nr_pages)
+ break;
+
+ pages[p++] = page;
+ }
+ s++;
+ }
+
+ err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);
+
+out:
+ kfree(pages);
+
+ return err;
+}
+#else
+static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
+ struct vm_area_struct *vma)
+{
+ return -EOPNOTSUPP;
+}
+#endif
+
+int ring_buffer_map(struct trace_buffer *buffer, int cpu,
+ struct vm_area_struct *vma)
+{
+ struct ring_buffer_per_cpu *cpu_buffer;
+ unsigned long flags, *subbuf_ids;
+ int err = 0;
+
+ if (!cpumask_test_cpu(cpu, buffer->cpumask))
+ return -EINVAL;
+
+ cpu_buffer = buffer->buffers[cpu];
+
+ mutex_lock(&cpu_buffer->mapping_lock);
+
+ if (cpu_buffer->mapped) {
+ err = __rb_map_vma(cpu_buffer, vma);
+ if (!err)
+ err = __rb_inc_dec_mapped(cpu_buffer, true);
+ mutex_unlock(&cpu_buffer->mapping_lock);
+ return err;
+ }
+
+ /* prevent another thread from changing buffer/sub-buffer sizes */
+ mutex_lock(&buffer->mutex);
+
+ err = rb_alloc_meta_page(cpu_buffer);
+ if (err)
+ goto unlock;
+
+ /* subbuf_ids include the reader while nr_pages does not */
+ subbuf_ids = kcalloc(cpu_buffer->nr_pages + 1, sizeof(*subbuf_ids), GFP_KERNEL);
+ if (!subbuf_ids) {
+ rb_free_meta_page(cpu_buffer);
+ err = -ENOMEM;
+ goto unlock;
+ }
+
+ atomic_inc(&cpu_buffer->resize_disabled);
+
+ /*
+ * Lock all readers to block any subbuf swap until the subbuf IDs are
+ * assigned.
+ */
+ raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+ rb_setup_ids_meta_page(cpu_buffer, subbuf_ids);
+ raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+
+ err = __rb_map_vma(cpu_buffer, vma);
+ if (!err) {
+ raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+ cpu_buffer->mapped = 1;
+ raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+ } else {
+ kfree(cpu_buffer->subbuf_ids);
+ cpu_buffer->subbuf_ids = NULL;
+ rb_free_meta_page(cpu_buffer);
+ }
+
+unlock:
+ mutex_unlock(&buffer->mutex);
+ mutex_unlock(&cpu_buffer->mapping_lock);
+
+ return err;
+}
+
+int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
+{
+ struct ring_buffer_per_cpu *cpu_buffer;
+ unsigned long flags;
+ int err = 0;
+
+ if (!cpumask_test_cpu(cpu, buffer->cpumask))
+ return -EINVAL;
+
+ cpu_buffer = buffer->buffers[cpu];
+
+ mutex_lock(&cpu_buffer->mapping_lock);
+
+ if (!cpu_buffer->mapped) {
+ err = -ENODEV;
+ goto out;
+ } else if (cpu_buffer->mapped > 1) {
+ __rb_inc_dec_mapped(cpu_buffer, false);
+ goto out;
+ }
+
+ mutex_lock(&buffer->mutex);
+ raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+
+ cpu_buffer->mapped = 0;
+
+ raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+
+ kfree(cpu_buffer->subbuf_ids);
+ cpu_buffer->subbuf_ids = NULL;
+ rb_free_meta_page(cpu_buffer);
+ atomic_dec(&cpu_buffer->resize_disabled);
+
+ mutex_unlock(&buffer->mutex);
+
+out:
+ mutex_unlock(&cpu_buffer->mapping_lock);
+
+ return err;
+}
+
+int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
+{
+ struct ring_buffer_per_cpu *cpu_buffer;
+ unsigned long reader_size;
+ unsigned long flags;
+
+ cpu_buffer = rb_get_mapped_buffer(buffer, cpu);
+ if (IS_ERR(cpu_buffer))
+ return (int)PTR_ERR(cpu_buffer);
+
+ raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+
+consume:
+ if (rb_per_cpu_empty(cpu_buffer))
+ goto out;
+
+ reader_size = rb_page_size(cpu_buffer->reader_page);
+
+ /*
+ * There are data to be read on the current reader page, we can
+ * return to the caller. But before that, we assume the latter will read
+ * everything. Let's update the kernel reader accordingly.
+ */
+ if (cpu_buffer->reader_page->read < reader_size) {
+ while (cpu_buffer->reader_page->read < reader_size)
+ rb_advance_reader(cpu_buffer);
+ goto out;
+ }
+
+ if (WARN_ON(!rb_get_reader_page(cpu_buffer)))
+ goto out;
+
+ goto consume;
+
+out:
+ /* Some archs do not have data cache coherency between kernel and user-space */
+ flush_dcache_folio(virt_to_folio(cpu_buffer->reader_page->page));
+
+ rb_update_meta_page(cpu_buffer);
+
+ raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+ rb_put_mapped_buffer(cpu_buffer);
+
+ return 0;
+}
+
/*
* We only allocate new buffers, never free them if the CPU goes down.
* If we were to free the buffer, then the user would lose any trace that was in
--
2.44.0.769.g3c40516874-goog
Currently, user-space extracts data from the ring-buffer via splice,
which is handy for storage or network sharing. However, due to splice
limitations, it is imposible to do real-time analysis without a copy.
A solution for that problem is to let the user-space map the ring-buffer
directly.
The mapping is exposed via the per-CPU file trace_pipe_raw. The first
element of the mapping is the meta-page. It is followed by each
subbuffer constituting the ring-buffer, ordered by their unique page ID:
* Meta-page -- include/uapi/linux/trace_mmap.h for a description
* Subbuf ID 0
* Subbuf ID 1
...
It is therefore easy to translate a subbuf ID into an offset in the
mapping:
reader_id = meta->reader->id;
reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;
When new data is available, the mapper must call a newly introduced ioctl:
TRACE_MMAP_IOCTL_GET_READER. This will update the Meta-page reader ID to
point to the next reader containing unread data.
Mapping will prevent snapshot and buffer size modifications.
CC: <[email protected]>
Signed-off-by: Vincent Donnefort <[email protected]>
diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
index b682e9925539..bd1066754220 100644
--- a/include/uapi/linux/trace_mmap.h
+++ b/include/uapi/linux/trace_mmap.h
@@ -43,4 +43,6 @@ struct trace_buffer_meta {
__u64 Reserved2;
};
+#define TRACE_MMAP_IOCTL_GET_READER _IO('T', 0x1)
+
#endif /* _TRACE_MMAP_H_ */
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 233d1af39fff..a35e7f598233 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1191,6 +1191,12 @@ static void tracing_snapshot_instance_cond(struct trace_array *tr,
return;
}
+ if (tr->mapped) {
+ trace_array_puts(tr, "*** BUFFER MEMORY MAPPED ***\n");
+ trace_array_puts(tr, "*** Can not use snapshot (sorry) ***\n");
+ return;
+ }
+
local_irq_save(flags);
update_max_tr(tr, current, smp_processor_id(), cond_data);
local_irq_restore(flags);
@@ -1323,7 +1329,7 @@ static int tracing_arm_snapshot_locked(struct trace_array *tr)
lockdep_assert_held(&trace_types_lock);
spin_lock(&tr->snapshot_trigger_lock);
- if (tr->snapshot == UINT_MAX) {
+ if (tr->snapshot == UINT_MAX || tr->mapped) {
spin_unlock(&tr->snapshot_trigger_lock);
return -EBUSY;
}
@@ -6068,7 +6074,7 @@ static void tracing_set_nop(struct trace_array *tr)
{
if (tr->current_trace == &nop_trace)
return;
-
+
tr->current_trace->enabled--;
if (tr->current_trace->reset)
@@ -8194,15 +8200,32 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
return ret;
}
-/* An ioctl call with cmd 0 to the ring buffer file will wake up all waiters */
static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct ftrace_buffer_info *info = file->private_data;
struct trace_iterator *iter = &info->iter;
+ int err;
+
+ if (cmd == TRACE_MMAP_IOCTL_GET_READER) {
+ if (!(file->f_flags & O_NONBLOCK)) {
+ err = ring_buffer_wait(iter->array_buffer->buffer,
+ iter->cpu_file,
+ iter->tr->buffer_percent,
+ NULL, NULL);
+ if (err)
+ return err;
+ }
- if (cmd)
- return -ENOIOCTLCMD;
+ return ring_buffer_map_get_reader(iter->array_buffer->buffer,
+ iter->cpu_file);
+ } else if (cmd) {
+ return -ENOTTY;
+ }
+ /*
+ * An ioctl call with cmd 0 to the ring buffer file will wake up all
+ * waiters
+ */
mutex_lock(&trace_types_lock);
/* Make sure the waiters see the new wait_index */
@@ -8214,6 +8237,76 @@ static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned
return 0;
}
+#ifdef CONFIG_TRACER_MAX_TRACE
+static int get_snapshot_map(struct trace_array *tr)
+{
+ int err = 0;
+
+ /*
+ * Called with mmap_lock held. lockdep would be unhappy if we would now
+ * take trace_types_lock. Instead use the specific
+ * snapshot_trigger_lock.
+ */
+ spin_lock(&tr->snapshot_trigger_lock);
+
+ if (tr->snapshot || tr->mapped == UINT_MAX)
+ err = -EBUSY;
+ else
+ tr->mapped++;
+
+ spin_unlock(&tr->snapshot_trigger_lock);
+
+ /* Wait for update_max_tr() to observe iter->tr->mapped */
+ if (tr->mapped == 1)
+ synchronize_rcu();
+
+ return err;
+
+}
+static void put_snapshot_map(struct trace_array *tr)
+{
+ spin_lock(&tr->snapshot_trigger_lock);
+ if (!WARN_ON(!tr->mapped))
+ tr->mapped--;
+ spin_unlock(&tr->snapshot_trigger_lock);
+}
+#else
+static inline int get_snapshot_map(struct trace_array *tr) { return 0; }
+static inline void put_snapshot_map(struct trace_array *tr) { }
+#endif
+
+static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
+{
+ struct ftrace_buffer_info *info = vma->vm_file->private_data;
+ struct trace_iterator *iter = &info->iter;
+
+ WARN_ON(ring_buffer_unmap(iter->array_buffer->buffer, iter->cpu_file));
+ put_snapshot_map(iter->tr);
+}
+
+static const struct vm_operations_struct tracing_buffers_vmops = {
+ .close = tracing_buffers_mmap_close,
+};
+
+static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
+{
+ struct ftrace_buffer_info *info = filp->private_data;
+ struct trace_iterator *iter = &info->iter;
+ int ret = 0;
+
+ ret = get_snapshot_map(iter->tr);
+ if (ret)
+ return ret;
+
+ ret = ring_buffer_map(iter->array_buffer->buffer, iter->cpu_file, vma);
+ if (ret)
+ put_snapshot_map(iter->tr);
+
+ vma->vm_ops = &tracing_buffers_vmops;
+
+ return ret;
+}
+
static const struct file_operations tracing_buffers_fops = {
.open = tracing_buffers_open,
.read = tracing_buffers_read,
@@ -8223,6 +8316,7 @@ static const struct file_operations tracing_buffers_fops = {
.splice_read = tracing_buffers_splice_read,
.unlocked_ioctl = tracing_buffers_ioctl,
.llseek = no_llseek,
+ .mmap = tracing_buffers_mmap,
};
static ssize_t
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 64450615ca0c..749a182dab48 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -336,6 +336,7 @@ struct trace_array {
bool allocated_snapshot;
spinlock_t snapshot_trigger_lock;
unsigned int snapshot;
+ unsigned int mapped;
unsigned long max_latency;
#ifdef CONFIG_FSNOTIFY
struct dentry *d_max_latency;
--
2.44.0.769.g3c40516874-goog
This test maps a ring-buffer and validate the meta-page after reset and
after emitting few events.
Cc: Shuah Khan <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: [email protected]
Acked-by: Muhammad Usama Anjum <[email protected]>
Signed-off-by: Vincent Donnefort <[email protected]>
diff --git a/tools/testing/selftests/ring-buffer/.gitignore b/tools/testing/selftests/ring-buffer/.gitignore
new file mode 100644
index 000000000000..3aed1a2a6c67
--- /dev/null
+++ b/tools/testing/selftests/ring-buffer/.gitignore
@@ -0,0 +1 @@
+map_test
diff --git a/tools/testing/selftests/ring-buffer/Makefile b/tools/testing/selftests/ring-buffer/Makefile
new file mode 100644
index 000000000000..627c5fa6d1ab
--- /dev/null
+++ b/tools/testing/selftests/ring-buffer/Makefile
@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-2.0
+CFLAGS += -Wl,-no-as-needed -Wall
+CFLAGS += $(KHDR_INCLUDES)
+CFLAGS += -D_GNU_SOURCE
+
+TEST_GEN_PROGS = map_test
+
+include ../lib.mk
diff --git a/tools/testing/selftests/ring-buffer/config b/tools/testing/selftests/ring-buffer/config
new file mode 100644
index 000000000000..d936f8f00e78
--- /dev/null
+++ b/tools/testing/selftests/ring-buffer/config
@@ -0,0 +1,2 @@
+CONFIG_FTRACE=y
+CONFIG_TRACER_SNAPSHOT=y
diff --git a/tools/testing/selftests/ring-buffer/map_test.c b/tools/testing/selftests/ring-buffer/map_test.c
new file mode 100644
index 000000000000..b95eedddbb8c
--- /dev/null
+++ b/tools/testing/selftests/ring-buffer/map_test.c
@@ -0,0 +1,288 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Ring-buffer memory mapping tests
+ *
+ * Copyright (c) 2024 Vincent Donnefort <[email protected]>
+ */
+#include <fcntl.h>
+#include <sched.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include <linux/trace_mmap.h>
+
+#include <sys/mman.h>
+#include <sys/ioctl.h>
+
+#include "../user_events/user_events_selftests.h" /* share tracefs setup */
+#include "../kselftest_harness.h"
+
+#define TRACEFS_ROOT "/sys/kernel/tracing"
+
+static int __tracefs_write(const char *path, const char *value)
+{
+ int fd, ret;
+
+ fd = open(path, O_WRONLY | O_TRUNC);
+ if (fd < 0)
+ return fd;
+
+ ret = write(fd, value, strlen(value));
+
+ close(fd);
+
+ return ret == -1 ? -errno : 0;
+}
+
+static int __tracefs_write_int(const char *path, int value)
+{
+ char *str;
+ int ret;
+
+ if (asprintf(&str, "%d", value) < 0)
+ return -1;
+
+ ret = __tracefs_write(path, str);
+
+ free(str);
+
+ return ret;
+}
+
+#define tracefs_write_int(path, value) \
+ ASSERT_EQ(__tracefs_write_int((path), (value)), 0)
+
+#define tracefs_write(path, value) \
+ ASSERT_EQ(__tracefs_write((path), (value)), 0)
+
+static int tracefs_reset(void)
+{
+ if (__tracefs_write_int(TRACEFS_ROOT"/tracing_on", 0))
+ return -1;
+ if (__tracefs_write(TRACEFS_ROOT"/trace", ""))
+ return -1;
+ if (__tracefs_write(TRACEFS_ROOT"/set_event", ""))
+ return -1;
+ if (__tracefs_write(TRACEFS_ROOT"/current_tracer", "nop"))
+ return -1;
+
+ return 0;
+}
+
+struct tracefs_cpu_map_desc {
+ struct trace_buffer_meta *meta;
+ int cpu_fd;
+};
+
+int tracefs_cpu_map(struct tracefs_cpu_map_desc *desc, int cpu)
+{
+ int page_size = getpagesize();
+ char *cpu_path;
+ void *map;
+
+ if (asprintf(&cpu_path,
+ TRACEFS_ROOT"/per_cpu/cpu%d/trace_pipe_raw",
+ cpu) < 0)
+ return -ENOMEM;
+
+ desc->cpu_fd = open(cpu_path, O_RDONLY | O_NONBLOCK);
+ free(cpu_path);
+ if (desc->cpu_fd < 0)
+ return -ENODEV;
+
+ map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, desc->cpu_fd, 0);
+ if (map == MAP_FAILED)
+ return -errno;
+
+ desc->meta = (struct trace_buffer_meta *)map;
+
+ return 0;
+}
+
+void tracefs_cpu_unmap(struct tracefs_cpu_map_desc *desc)
+{
+ munmap(desc->meta, desc->meta->meta_page_size);
+ close(desc->cpu_fd);
+}
+
+FIXTURE(map) {
+ struct tracefs_cpu_map_desc map_desc;
+ bool umount;
+};
+
+FIXTURE_VARIANT(map) {
+ int subbuf_size;
+};
+
+FIXTURE_VARIANT_ADD(map, subbuf_size_4k) {
+ .subbuf_size = 4,
+};
+
+FIXTURE_VARIANT_ADD(map, subbuf_size_8k) {
+ .subbuf_size = 8,
+};
+
+FIXTURE_SETUP(map)
+{
+ int cpu = sched_getcpu();
+ cpu_set_t cpu_mask;
+ bool fail, umount;
+ char *message;
+
+ if (!tracefs_enabled(&message, &fail, &umount)) {
+ if (fail) {
+ TH_LOG("Tracefs setup failed: %s", message);
+ ASSERT_FALSE(fail);
+ }
+ SKIP(return, "Skipping: %s", message);
+ }
+
+ self->umount = umount;
+
+ ASSERT_GE(cpu, 0);
+
+ ASSERT_EQ(tracefs_reset(), 0);
+
+ tracefs_write_int(TRACEFS_ROOT"/buffer_subbuf_size_kb", variant->subbuf_size);
+
+ ASSERT_EQ(tracefs_cpu_map(&self->map_desc, cpu), 0);
+
+ /*
+ * Ensure generated events will be found on this very same ring-buffer.
+ */
+ CPU_ZERO(&cpu_mask);
+ CPU_SET(cpu, &cpu_mask);
+ ASSERT_EQ(sched_setaffinity(0, sizeof(cpu_mask), &cpu_mask), 0);
+}
+
+FIXTURE_TEARDOWN(map)
+{
+ tracefs_reset();
+
+ if (self->umount)
+ tracefs_unmount();
+
+ tracefs_cpu_unmap(&self->map_desc);
+}
+
+TEST_F(map, meta_page_check)
+{
+ struct tracefs_cpu_map_desc *desc = &self->map_desc;
+ int cnt = 0;
+
+ ASSERT_EQ(desc->meta->entries, 0);
+ ASSERT_EQ(desc->meta->overrun, 0);
+ ASSERT_EQ(desc->meta->read, 0);
+
+ ASSERT_EQ(desc->meta->reader.id, 0);
+ ASSERT_EQ(desc->meta->reader.read, 0);
+
+ ASSERT_EQ(ioctl(desc->cpu_fd, TRACE_MMAP_IOCTL_GET_READER), 0);
+ ASSERT_EQ(desc->meta->reader.id, 0);
+
+ tracefs_write_int(TRACEFS_ROOT"/tracing_on", 1);
+ for (int i = 0; i < 16; i++)
+ tracefs_write_int(TRACEFS_ROOT"/trace_marker", i);
+again:
+ ASSERT_EQ(ioctl(desc->cpu_fd, TRACE_MMAP_IOCTL_GET_READER), 0);
+
+ ASSERT_EQ(desc->meta->entries, 16);
+ ASSERT_EQ(desc->meta->overrun, 0);
+ ASSERT_EQ(desc->meta->read, 16);
+
+ ASSERT_EQ(desc->meta->reader.id, 1);
+
+ if (!(cnt++))
+ goto again;
+}
+
+TEST_F(map, data_mmap)
+{
+ struct tracefs_cpu_map_desc *desc = &self->map_desc;
+ unsigned long meta_len, data_len;
+ void *data;
+
+ meta_len = desc->meta->meta_page_size;
+ data_len = desc->meta->subbuf_size * desc->meta->nr_subbufs;
+
+ /* Map all the available subbufs */
+ data = mmap(NULL, data_len, PROT_READ, MAP_SHARED,
+ desc->cpu_fd, meta_len);
+ ASSERT_NE(data, MAP_FAILED);
+ munmap(data, data_len);
+
+ /* Map all the available subbufs - 1 */
+ data_len -= desc->meta->subbuf_size;
+ data = mmap(NULL, data_len, PROT_READ, MAP_SHARED,
+ desc->cpu_fd, meta_len);
+ ASSERT_NE(data, MAP_FAILED);
+ munmap(data, data_len);
+
+ /* Overflow the available subbufs by 1 */
+ meta_len += desc->meta->subbuf_size * 2;
+ data = mmap(NULL, data_len, PROT_READ, MAP_SHARED,
+ desc->cpu_fd, meta_len);
+ ASSERT_EQ(data, MAP_FAILED);
+}
+
+FIXTURE(snapshot) {
+ bool umount;
+};
+
+FIXTURE_SETUP(snapshot)
+{
+ bool fail, umount;
+ struct stat sb;
+ char *message;
+
+ if (stat(TRACEFS_ROOT"/snapshot", &sb))
+ SKIP(return, "Skipping: %s", "snapshot not available");
+
+ if (!tracefs_enabled(&message, &fail, &umount)) {
+ if (fail) {
+ TH_LOG("Tracefs setup failed: %s", message);
+ ASSERT_FALSE(fail);
+ }
+ SKIP(return, "Skipping: %s", message);
+ }
+
+ self->umount = umount;
+}
+
+FIXTURE_TEARDOWN(snapshot)
+{
+ __tracefs_write(TRACEFS_ROOT"/events/sched/sched_switch/trigger",
+ "!snapshot");
+ tracefs_reset();
+
+ if (self->umount)
+ tracefs_unmount();
+}
+
+TEST_F(snapshot, excludes_map)
+{
+ struct tracefs_cpu_map_desc map_desc;
+ int cpu = sched_getcpu();
+
+ ASSERT_GE(cpu, 0);
+ tracefs_write(TRACEFS_ROOT"/events/sched/sched_switch/trigger",
+ "snapshot");
+ ASSERT_EQ(tracefs_cpu_map(&map_desc, cpu), -EBUSY);
+}
+
+TEST_F(snapshot, excluded_by_map)
+{
+ struct tracefs_cpu_map_desc map_desc;
+ int cpu = sched_getcpu();
+
+ ASSERT_EQ(tracefs_cpu_map(&map_desc, cpu), 0);
+
+ ASSERT_EQ(__tracefs_write(TRACEFS_ROOT"/events/sched/sched_switch/trigger",
+ "snapshot"), -EBUSY);
+ ASSERT_EQ(__tracefs_write(TRACEFS_ROOT"/snapshot",
+ "1"), -EBUSY);
+}
+
+TEST_HARNESS_MAIN
--
2.44.0.769.g3c40516874-goog
It is now possible to mmap() a ring-buffer to stream its content. Add
some documentation and a code example.
Signed-off-by: Vincent Donnefort <[email protected]>
diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst
index 5092d6c13af5..0b300901fd75 100644
--- a/Documentation/trace/index.rst
+++ b/Documentation/trace/index.rst
@@ -29,6 +29,7 @@ Linux Tracing Technologies
timerlat-tracer
intel_th
ring-buffer-design
+ ring-buffer-map
stm
sys-t
coresight/index
diff --git a/Documentation/trace/ring-buffer-map.rst b/Documentation/trace/ring-buffer-map.rst
new file mode 100644
index 000000000000..8e296bcc0d7f
--- /dev/null
+++ b/Documentation/trace/ring-buffer-map.rst
@@ -0,0 +1,106 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Tracefs ring-buffer memory mapping
+==================================
+
+:Author: Vincent Donnefort <[email protected]>
+
+Overview
+========
+Tracefs ring-buffer memory map provides an efficient method to stream data
+as no memory copy is necessary. The application mapping the ring-buffer becomes
+then a consumer for that ring-buffer, in a similar fashion to trace_pipe.
+
+Memory mapping setup
+====================
+The mapping works with a mmap() of the trace_pipe_raw interface.
+
+The first system page of the mapping contains ring-buffer statistics and
+description. It is referred to as the meta-page. One of the most important
+fields of the meta-page is the reader. It contains the sub-buffer ID which can
+be safely read by the mapper (see ring-buffer-design.rst).
+
+The meta-page is followed by all the sub-buffers, ordered by ascending ID. It is
+therefore effortless to know where the reader starts in the mapping:
+
+.. code-block:: c
+
+ reader_id = meta->reader->id;
+ reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;
+
+When the application is done with the current reader, it can get a new one using
+the trace_pipe_raw ioctl() TRACE_MMAP_IOCTL_GET_READER. This ioctl also updates
+the meta-page fields.
+
+Limitations
+===========
+When a mapping is in place on a Tracefs ring-buffer, it is not possible to
+either resize it (either by increasing the entire size of the ring-buffer or
+each subbuf). It is also not possible to use snapshot and causes splice to copy
+the ring buffer data instead of using the copyless swap from the ring buffer.
+
+Concurrent readers (either another application mapping that ring-buffer or the
+kernel with trace_pipe) are allowed but not recommended. They will compete for
+the ring-buffer and the output is unpredictable, just like concurrent readers on
+trace_pipe would be.
+
+Example
+=======
+
+.. code-block:: c
+
+ #include <fcntl.h>
+ #include <stdio.h>
+ #include <stdlib.h>
+ #include <unistd.h>
+
+ #include <linux/trace_mmap.h>
+
+ #include <sys/mman.h>
+ #include <sys/ioctl.h>
+
+ #define TRACE_PIPE_RAW "/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw"
+
+ int main(void)
+ {
+ int page_size = getpagesize(), fd, reader_id;
+ unsigned long meta_len, data_len;
+ struct trace_buffer_meta *meta;
+ void *map, *reader, *data;
+
+ fd = open(TRACE_PIPE_RAW, O_RDONLY | O_NONBLOCK);
+ if (fd < 0)
+ exit(EXIT_FAILURE);
+
+ map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
+ if (map == MAP_FAILED)
+ exit(EXIT_FAILURE);
+
+ meta = (struct trace_buffer_meta *)map;
+ meta_len = meta->meta_page_size;
+
+ printf("entries: %llu\n", meta->entries);
+ printf("overrun: %llu\n", meta->overrun);
+ printf("read: %llu\n", meta->read);
+ printf("nr_subbufs: %u\n", meta->nr_subbufs);
+
+ data_len = meta->subbuf_size * meta->nr_subbufs;
+ data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, meta_len);
+ if (data == MAP_FAILED)
+ exit(EXIT_FAILURE);
+
+ if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER) < 0)
+ exit(EXIT_FAILURE);
+
+ reader_id = meta->reader.id;
+ reader = data + meta->subbuf_size * reader_id;
+
+ printf("Current reader address: %p\n", reader);
+
+ munmap(data, data_len);
+ munmap(meta, meta_len);
+ close (fd);
+
+ return 0;
+ }
--
2.44.0.769.g3c40516874-goog
I gave it some more thought, and I think we are still missing something
(I wish PFNMAP/MIXEDMAP wouldn't be that hard).
> +
> +/*
> + * +--------------+ pgoff == 0
> + * | meta page |
> + * +--------------+ pgoff == 1
> + * | subbuffer 0 |
> + * | |
> + * +--------------+ pgoff == (1 + (1 << subbuf_order))
> + * | subbuffer 1 |
> + * | |
> + * ...
> + */
> +#ifdef CONFIG_MMU
> +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
> + struct vm_area_struct *vma)
> +{
> + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
> + unsigned int subbuf_pages, subbuf_order;
> + struct page **pages;
> + int p = 0, s = 0;
> + int err;
> +
I'd add some comments here like
/* Refuse any MAP_PRIVATE or writable mappings. */
> + if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
> + !(vma->vm_flags & VM_MAYSHARE))
> + return -EPERM;
> +
/*
* Make sure the mapping cannot become writable later. Also, tell the VM
* to not touch these pages pages (VM_DONTCOPY | VM_DONTDUMP) and tell
* GUP to leave them alone as well (VM_IO).
*/
> + vm_flags_mod(vma,
> + VM_MIXEDMAP | VM_PFNMAP |
> + VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
> + VM_MAYWRITE);
I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all
and, as stated, vm_insert_pages() even complains quite a lot when it
would have to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a
very good reason.
Can't we limit ourselves to VM_IO?
But then, I wonder if it really helps much regarding GUP: yes, it blocks
ordinary GUP (see check_vma_flags()) but as
insert_page_into_pte_locked() does *not* set pte_special(), GUP-fast
(gup_fast_pte_range()) will not reject it.
Really, if you want GUP-fast to reject it, remap_pfn_range() and friends
are the way to go, that will set pte_special() such that also GUP-fast
will leave it alone, just like vm_normal_page() would.
So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it
alone won't stop all of GUP. We really have to mark the PTE as special,
which vm_insert_page() must not do (because it is refcounted!).
Which means: do we really have to stop GUP from grabbing that page?
Using vm_insert_page() only with VM_MIXEDMAP (and without
VM_PFNMAP|VM_IO) would be better.
If we want to stop all of GUP, remap_pfn_range() currently seems
unavoidable :(
> +
> + lockdep_assert_held(&cpu_buffer->mapping_lock);
> +
> + subbuf_order = cpu_buffer->buffer->subbuf_order;
> + subbuf_pages = 1 << subbuf_order;
> +
> + nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
> + nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
> +
> + vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
> + if (!vma_pages || vma_pages > nr_pages)
> + return -EINVAL;
> +
> + nr_pages = vma_pages;
> +
> + pages = kcalloc(nr_pages, sizeof(*pages), GFP_KERNEL);
> + if (!pages)
> + return -ENOMEM;
> +
> + if (!pgoff) {
> + pages[p++] = virt_to_page(cpu_buffer->meta_page);
> +
> + /*
> + * TODO: Align sub-buffers on their size, once
> + * vm_insert_pages() supports the zero-page.
> + */
> + } else {
> + /* Skip the meta-page */
> + pgoff--;
> +
> + if (pgoff % subbuf_pages) {
> + err = -EINVAL;
> + goto out;
> + }
> +
> + s += pgoff / subbuf_pages;
> + }
> +
> + while (s < nr_subbufs && p < nr_pages) {
> + struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);
> + int off = 0;
> +
> + for (; off < (1 << (subbuf_order)); off++, page++) {
> + if (p >= nr_pages)
> + break;
> +
> + pages[p++] = page;
> + }
> + s++;
> + }
> +
> + err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);
vm_insert_pages() documents: "In case of error, we may have mapped a
subset of the provided pages. It is the caller's responsibility to
account for this case."
Which could for example happen, when allocating a page table fails.
Would we able to deal with that here?
Again, I wish it would all be easier ...
--
Cheers,
David / dhildenb
Hi David,
Thanks for your quick response.
On Wed, Apr 24, 2024 at 05:26:39PM +0200, David Hildenbrand wrote:
>
> I gave it some more thought, and I think we are still missing something (I
> wish PFNMAP/MIXEDMAP wouldn't be that hard).
>
> > +
> > +/*
> > + * +--------------+ pgoff == 0
> > + * | meta page |
> > + * +--------------+ pgoff == 1
> > + * | subbuffer 0 |
> > + * | |
> > + * +--------------+ pgoff == (1 + (1 << subbuf_order))
> > + * | subbuffer 1 |
> > + * | |
> > + * ...
> > + */
> > +#ifdef CONFIG_MMU
> > +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
> > + struct vm_area_struct *vma)
> > +{
> > + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
> > + unsigned int subbuf_pages, subbuf_order;
> > + struct page **pages;
> > + int p = 0, s = 0;
> > + int err;
> > +
>
> I'd add some comments here like
>
> /* Refuse any MAP_PRIVATE or writable mappings. */
> > + if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
> > + !(vma->vm_flags & VM_MAYSHARE))
> > + return -EPERM;
> > +
>
> /*
> * Make sure the mapping cannot become writable later. Also, tell the VM
> * to not touch these pages pages (VM_DONTCOPY | VM_DONTDUMP) and tell
> * GUP to leave them alone as well (VM_IO).
> */
> > + vm_flags_mod(vma,
> > + VM_MIXEDMAP | VM_PFNMAP |
> > + VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
> > + VM_MAYWRITE);
>
> I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all and,
> as stated, vm_insert_pages() even complains quite a lot when it would have
> to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a very good
> reason.
>
> Can't we limit ourselves to VM_IO?
>
> But then, I wonder if it really helps much regarding GUP: yes, it blocks
> ordinary GUP (see check_vma_flags()) but as insert_page_into_pte_locked()
> does *not* set pte_special(), GUP-fast (gup_fast_pte_range()) will not
> reject it.
>
> Really, if you want GUP-fast to reject it, remap_pfn_range() and friends are
> the way to go, that will set pte_special() such that also GUP-fast will
> leave it alone, just like vm_normal_page() would.
>
> So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it alone
> won't stop all of GUP. We really have to mark the PTE as special, which
> vm_insert_page() must not do (because it is refcounted!).
Hum, apologies, I am not sure to follow the connection here. Why do you think
the recommendation was to prevent GUP?
>
> Which means: do we really have to stop GUP from grabbing that page?
>
> Using vm_insert_page() only with VM_MIXEDMAP (and without VM_PFNMAP|VM_IO)
> would be better.
Under the assumption we do not want to stop all GUP, why not using VM_IO over
VM_MIXEDMAP which is I believe more restrictive?
>
> If we want to stop all of GUP, remap_pfn_range() currently seems unavoidable
> :(
>
>
> > +
> > + lockdep_assert_held(&cpu_buffer->mapping_lock);
> > +
> > + subbuf_order = cpu_buffer->buffer->subbuf_order;
> > + subbuf_pages = 1 << subbuf_order;
> > +
> > + nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
> > + nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
> > +
> > + vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
> > + if (!vma_pages || vma_pages > nr_pages)
> > + return -EINVAL;
> > +
> > + nr_pages = vma_pages;
> > +
> > + pages = kcalloc(nr_pages, sizeof(*pages), GFP_KERNEL);
> > + if (!pages)
> > + return -ENOMEM;
> > +
> > + if (!pgoff) {
> > + pages[p++] = virt_to_page(cpu_buffer->meta_page);
> > +
> > + /*
> > + * TODO: Align sub-buffers on their size, once
> > + * vm_insert_pages() supports the zero-page.
> > + */
> > + } else {
> > + /* Skip the meta-page */
> > + pgoff--;
> > +
> > + if (pgoff % subbuf_pages) {
> > + err = -EINVAL;
> > + goto out;
> > + }
> > +
> > + s += pgoff / subbuf_pages;
> > + }
> > +
> > + while (s < nr_subbufs && p < nr_pages) {
> > + struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);
> > + int off = 0;
> > +
> > + for (; off < (1 << (subbuf_order)); off++, page++) {
> > + if (p >= nr_pages)
> > + break;
> > +
> > + pages[p++] = page;
> > + }
> > + s++;
> > + }
> > +
> > + err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);
>
> vm_insert_pages() documents: "In case of error, we may have mapped a subset
> of the provided pages. It is the caller's responsibility to account for this
> case."
>
> Which could for example happen, when allocating a page table fails.
>
> Would we able to deal with that here?
As we are in the mmap path, on an error, I would expect the vma to be destroyed
and those pages whom insertion succeeded to be unmapped?
But perhaps shall we proactively zap_page_range_single()?
>
>
> Again, I wish it would all be easier ...
>
> --
> Cheers,
>
> David / dhildenb
>
> To unsubscribe from this group and stop receiving emails from it, send an email to [email protected].
>
On 24.04.24 22:31, Vincent Donnefort wrote:
> Hi David,
>
> Thanks for your quick response.
>
> On Wed, Apr 24, 2024 at 05:26:39PM +0200, David Hildenbrand wrote:
>>
>> I gave it some more thought, and I think we are still missing something (I
>> wish PFNMAP/MIXEDMAP wouldn't be that hard).
>>
>>> +
>>> +/*
>>> + * +--------------+ pgoff == 0
>>> + * | meta page |
>>> + * +--------------+ pgoff == 1
>>> + * | subbuffer 0 |
>>> + * | |
>>> + * +--------------+ pgoff == (1 + (1 << subbuf_order))
>>> + * | subbuffer 1 |
>>> + * | |
>>> + * ...
>>> + */
>>> +#ifdef CONFIG_MMU
>>> +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
>>> + struct vm_area_struct *vma)
>>> +{
>>> + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
>>> + unsigned int subbuf_pages, subbuf_order;
>>> + struct page **pages;
>>> + int p = 0, s = 0;
>>> + int err;
>>> +
>>
>> I'd add some comments here like
>>
>> /* Refuse any MAP_PRIVATE or writable mappings. */
>>> + if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
>>> + !(vma->vm_flags & VM_MAYSHARE))
>>> + return -EPERM;
>>> +
>>
>> /*
>> * Make sure the mapping cannot become writable later. Also, tell the VM
>> * to not touch these pages pages (VM_DONTCOPY | VM_DONTDUMP) and tell
>> * GUP to leave them alone as well (VM_IO).
>> */
>>> + vm_flags_mod(vma,
>>> + VM_MIXEDMAP | VM_PFNMAP |
>>> + VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
>>> + VM_MAYWRITE);
>>
>> I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all and,
>> as stated, vm_insert_pages() even complains quite a lot when it would have
>> to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a very good
>> reason.
>>
>> Can't we limit ourselves to VM_IO?
>>
>> But then, I wonder if it really helps much regarding GUP: yes, it blocks
>> ordinary GUP (see check_vma_flags()) but as insert_page_into_pte_locked()
>> does *not* set pte_special(), GUP-fast (gup_fast_pte_range()) will not
>> reject it.
>>
>> Really, if you want GUP-fast to reject it, remap_pfn_range() and friends are
>> the way to go, that will set pte_special() such that also GUP-fast will
>> leave it alone, just like vm_normal_page() would.
>>
>> So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it alone
>> won't stop all of GUP. We really have to mark the PTE as special, which
>> vm_insert_page() must not do (because it is refcounted!).
>
> Hum, apologies, I am not sure to follow the connection here. Why do you think
> the recommendation was to prevent GUP?
Ah, I'm hallucinating! :) "not let people play games with the mapping" to me
implied "make sure nobody touches it". If GUP is acceptable that makes stuff
a lot easier. VM_IO will block some GUP, but not all of it.
>
>>
>> Which means: do we really have to stop GUP from grabbing that page?
>>
>> Using vm_insert_page() only with VM_MIXEDMAP (and without VM_PFNMAP|VM_IO)
>> would be better.
>
> Under the assumption we do not want to stop all GUP, why not using VM_IO over
> VM_MIXEDMAP which is I believe more restrictive?
VM_MIXEDMAP will be implicitly set by vm_insert_page(). There is a lengthy comment
for vm_normal_page() that explains all this madness. VM_MIXEDMAP is primarily
relevant for COW mappings, which you just forbid completely.
remap_pfn_range_notrack() documents the semantics of some of the other flags:
* VM_IO tells people not to look at these pages
* (accesses can have side effects).
* VM_PFNMAP tells the core MM that the base pages are just
* raw PFN mappings, and do not have a "struct page" associated
* with them.
* VM_DONTEXPAND
* Disable vma merging and expanding with mremap().
* VM_DONTDUMP
* Omit vma from core dump, even when VM_IO turned off.
VM_PFNMAP is very likely really not what we want, unless we really perform raw
PFN mappings ... VM_IO we can set without doing much harm.
So I would suggest dropping VM_PFNMAP when using vm_insert_pages(), using only VM_IO
and likely just letting vm_insert_pages() set VM_MIXEDMAP for you.
[...]
>>
>> vm_insert_pages() documents: "In case of error, we may have mapped a subset
>> of the provided pages. It is the caller's responsibility to account for this
>> case."
>>
>> Which could for example happen, when allocating a page table fails.
>>
>> Would we able to deal with that here?
>
> As we are in the mmap path, on an error, I would expect the vma to be destroyed
> and those pages whom insertion succeeded to be unmapped?
>
Ah, we simply fail ->mmap().
In mmap_region(), if call_mmap() failed, we "goto unmap_and_free_vma" where we have
/* Undo any partial mapping done by a device driver. */
unmap_region(mm, &vmi.mas, vma, prev, next, vma->vm_start, vma->vm_end, vma->vm_end, true);
> But perhaps shall we proactively zap_page_range_single()?
No mmap_region() should indeed be handling it correctly already!
--
Cheers,
David / dhildenb
On Wed, Apr 24, 2024 at 10:55:54PM +0200, David Hildenbrand wrote:
> On 24.04.24 22:31, Vincent Donnefort wrote:
> > Hi David,
> >
> > Thanks for your quick response.
> >
> > On Wed, Apr 24, 2024 at 05:26:39PM +0200, David Hildenbrand wrote:
> > >
> > > I gave it some more thought, and I think we are still missing something (I
> > > wish PFNMAP/MIXEDMAP wouldn't be that hard).
> > >
> > > > +
> > > > +/*
> > > > + * +--------------+ pgoff == 0
> > > > + * | meta page |
> > > > + * +--------------+ pgoff == 1
> > > > + * | subbuffer 0 |
> > > > + * | |
> > > > + * +--------------+ pgoff == (1 + (1 << subbuf_order))
> > > > + * | subbuffer 1 |
> > > > + * | |
> > > > + * ...
> > > > + */
> > > > +#ifdef CONFIG_MMU
> > > > +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
> > > > + struct vm_area_struct *vma)
> > > > +{
> > > > + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
> > > > + unsigned int subbuf_pages, subbuf_order;
> > > > + struct page **pages;
> > > > + int p = 0, s = 0;
> > > > + int err;
> > > > +
> > >
> > > I'd add some comments here like
> > >
> > > /* Refuse any MAP_PRIVATE or writable mappings. */
> > > > + if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
> > > > + !(vma->vm_flags & VM_MAYSHARE))
> > > > + return -EPERM;
> > > > +
> > >
> > > /*
> > > * Make sure the mapping cannot become writable later. Also, tell the VM
> > > * to not touch these pages pages (VM_DONTCOPY | VM_DONTDUMP) and tell
> > > * GUP to leave them alone as well (VM_IO).
> > > */
> > > > + vm_flags_mod(vma,
> > > > + VM_MIXEDMAP | VM_PFNMAP |
> > > > + VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
> > > > + VM_MAYWRITE);
> > >
> > > I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all and,
> > > as stated, vm_insert_pages() even complains quite a lot when it would have
> > > to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a very good
> > > reason.
> > >
> > > Can't we limit ourselves to VM_IO?
> > >
> > > But then, I wonder if it really helps much regarding GUP: yes, it blocks
> > > ordinary GUP (see check_vma_flags()) but as insert_page_into_pte_locked()
> > > does *not* set pte_special(), GUP-fast (gup_fast_pte_range()) will not
> > > reject it.
> > >
> > > Really, if you want GUP-fast to reject it, remap_pfn_range() and friends are
> > > the way to go, that will set pte_special() such that also GUP-fast will
> > > leave it alone, just like vm_normal_page() would.
> > >
> > > So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it alone
> > > won't stop all of GUP. We really have to mark the PTE as special, which
> > > vm_insert_page() must not do (because it is refcounted!).
> >
> > Hum, apologies, I am not sure to follow the connection here. Why do you think
> > the recommendation was to prevent GUP?
>
> Ah, I'm hallucinating! :) "not let people play games with the mapping" to me
> implied "make sure nobody touches it". If GUP is acceptable that makes stuff
> a lot easier. VM_IO will block some GUP, but not all of it.
>
> >
> > >
> > > Which means: do we really have to stop GUP from grabbing that page?
> > >
> > > Using vm_insert_page() only with VM_MIXEDMAP (and without VM_PFNMAP|VM_IO)
> > > would be better.
> >
> > Under the assumption we do not want to stop all GUP, why not using VM_IO over
> > VM_MIXEDMAP which is I believe more restrictive?
>
> VM_MIXEDMAP will be implicitly set by vm_insert_page(). There is a lengthy comment
> for vm_normal_page() that explains all this madness. VM_MIXEDMAP is primarily
> relevant for COW mappings, which you just forbid completely.
>
> remap_pfn_range_notrack() documents the semantics of some of the other flags:
>
> * VM_IO tells people not to look at these pages
> * (accesses can have side effects).
> * VM_PFNMAP tells the core MM that the base pages are just
> * raw PFN mappings, and do not have a "struct page" associated
> * with them.
> * VM_DONTEXPAND
> * Disable vma merging and expanding with mremap().
> * VM_DONTDUMP
> * Omit vma from core dump, even when VM_IO turned off.
>
> VM_PFNMAP is very likely really not what we want, unless we really perform raw
> PFN mappings ... VM_IO we can set without doing much harm.
>
> So I would suggest dropping VM_PFNMAP when using vm_insert_pages(), using only VM_IO
> and likely just letting vm_insert_pages() set VM_MIXEDMAP for you.
Sounds good, I will do that in v22.
>
> [...]
>
> > >
> > > vm_insert_pages() documents: "In case of error, we may have mapped a subset
> > > of the provided pages. It is the caller's responsibility to account for this
> > > case."
> > >
> > > Which could for example happen, when allocating a page table fails.
> > >
> > > Would we able to deal with that here?
> >
> > As we are in the mmap path, on an error, I would expect the vma to be destroyed
> > and those pages whom insertion succeeded to be unmapped?
> >
>
> Ah, we simply fail ->mmap().
>
> In mmap_region(), if call_mmap() failed, we "goto unmap_and_free_vma" where we have
>
> /* Undo any partial mapping done by a device driver. */
> unmap_region(mm, &vmi.mas, vma, prev, next, vma->vm_start, vma->vm_end, vma->vm_end, true);
>
>
> > But perhaps shall we proactively zap_page_range_single()?
>
> No mmap_region() should indeed be handling it correctly already!
Ok, thanks for confirming!
>
> --
> Cheers,
>
> David / dhildenb
>