Hello,
here is the sixth version of my patches to implement synchronous page faults
for DAX mappings to make flushing of DAX mappings possible from userspace so
that they can be flushed on finer than page granularity and also avoid the
overhead of a syscall.
I think we are ready to get this merged - I've talked to Dan and he said he
could take the patches through his tree. It would just be nice to get final
ack from Christoph for the first patch implementing MAP_VALIDATE and someone
from XFS folks to check patch 17 (make xfs_filemap_pfn_mkwrite use
__xfs_filemap_fault()).
---
We use a new mmap flag MAP_SYNC to indicate that page faults for the mapping
should be synchronous. The guarantee provided by this flag is: While a block
is writeably mapped into page tables of this mapping, it is guaranteed to be
visible in the file at that offset also after a crash.
How I implement this is that ->iomap_begin() indicates by a flag that inode
block mapping metadata is unstable and may need flushing (use the same test as
whether fdatasync() has metadata to write). If yes, DAX fault handler refrains
from inserting / write-enabling the page table entry and returns special flag
VM_FAULT_NEEDDSYNC together with a PFN to map to the filesystem fault handler.
The handler then calls fdatasync() (vfs_fsync_range()) for the affected range
and after that calls DAX code to update the page table entry appropriately.
I did some basic performance testing on the patches over ramdisk - timed
latency of page faults when faulting 512 pages. I did several tests: with file
preallocated / with file empty, with background file copying going on / without
it, with / without MAP_SYNC (so that we get comparison). The results are
(numbers are in microseconds):
File preallocated, no background load no MAP_SYNC:
min=9 avg=10 max=46
8 - 15 us: 508
16 - 31 us: 3
32 - 63 us: 1
File preallocated, no background load, MAP_SYNC:
min=9 avg=10 max=47
8 - 15 us: 508
16 - 31 us: 2
32 - 63 us: 2
File empty, no background load, no MAP_SYNC:
min=21 avg=22 max=70
16 - 31 us: 506
32 - 63 us: 5
64 - 127 us: 1
File empty, no background load, MAP_SYNC:
min=40 avg=124 max=242
32 - 63 us: 1
64 - 127 us: 333
128 - 255 us: 178
File empty, background load, no MAP_SYNC:
min=21 avg=23 max=67
16 - 31 us: 507
32 - 63 us: 4
64 - 127 us: 1
File empty, background load, MAP_SYNC:
min=94 avg=112 max=181
64 - 127 us: 489
128 - 255 us: 23
So here we can see the difference between MAP_SYNC vs non MAP_SYNC is about
100-200 us when we need to wait for transaction commit in this setup.
Changes since v5:
* really updated the manpage
* improved comment describing IOMAP_F_DIRTY
* fixed XFS handling of VM_FAULT_NEEDSYNC in xfs_filemap_pfn_mkwrite()
Changes since v4:
* fixed couple of minor things in the manpage
* make legacy mmap flags always supported, remove them from mask declared
to be supported by ext4 and xfs
Changes since v3:
* updated some changelogs
* folded fs support for VM_SYNC flag into patches implementing the
functionality
* removed ->mmap_validate, use ->mmap_supported_flags instead
* added some Reviewed-by tags
* added manpage patch
Changes since v2:
* avoid unnecessary flushing of faulted page (Ross) - I've realized it makes no
sense to remeasure my benchmark results (after actually doing that and seeing
no difference, sigh) since I use ramdisk and not real PMEM HW and so flushes
are ignored.
* handle nojournal mode of ext4
* other smaller cleanups & fixes (Ross)
* factor larger part of finishing of synchronous fault into a helper (Christoph)
* reorder pfnp argument of dax_iomap_fault() (Christoph)
* add XFS support from Christoph
* use proper MAP_SYNC support in mmap(2)
* rebased on top of 4.14-rc4
Changes since v1:
* switched to using mmap flag MAP_SYNC
* cleaned up fault handlers to avoid passing pfn in vmf->orig_pte
* switched to not touching page tables before we are ready to insert final
entry as it was unnecessary and not really simplifying anything
* renamed fault flag to VM_FAULT_NEEDDSYNC
* other smaller fixes found by reviewers
Honza
It is unused.
Reviewed-by: Ross Zwisler <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
---
include/linux/mm.h | 2 --
1 file changed, 2 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 065d99deb847..ca72b67153d5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1182,8 +1182,6 @@ static inline void clear_page_pfmemalloc(struct page *page)
#define VM_FAULT_FALLBACK 0x0800 /* huge page fault failed, fall back to small */
#define VM_FAULT_DONE_COW 0x1000 /* ->fault has fully handled COW */
-#define VM_FAULT_HWPOISON_LARGE_MASK 0xf000 /* encodes hpage index for large hwpoison */
-
#define VM_FAULT_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \
VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
VM_FAULT_FALLBACK)
--
2.12.3
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
Factor out code to get pfn out of iomap that is shared between PTE and
PMD fault path.
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Ross Zwisler <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
---
fs/dax.c | 83 +++++++++++++++++++++++++++++++++-------------------------------
1 file changed, 43 insertions(+), 40 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index 0bc42ac294ca..116eef8d6c69 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -825,30 +825,53 @@ static sector_t dax_iomap_sector(struct iomap *iomap, loff_t pos)
return iomap->blkno + (((pos & PAGE_MASK) - iomap->offset) >> 9);
}
-static int dax_insert_mapping(struct vm_fault *vmf, struct iomap *iomap,
- loff_t pos, void *entry)
+static int dax_iomap_pfn(struct iomap *iomap, loff_t pos, size_t size,
+ pfn_t *pfnp)
{
const sector_t sector = dax_iomap_sector(iomap, pos);
- struct vm_area_struct *vma = vmf->vma;
- struct address_space *mapping = vma->vm_file->f_mapping;
- unsigned long vaddr = vmf->address;
- void *ret, *kaddr;
pgoff_t pgoff;
+ void *kaddr;
int id, rc;
- pfn_t pfn;
+ long length;
- rc = bdev_dax_pgoff(iomap->bdev, sector, PAGE_SIZE, &pgoff);
+ rc = bdev_dax_pgoff(iomap->bdev, sector, size, &pgoff);
if (rc)
return rc;
-
id = dax_read_lock();
- rc = dax_direct_access(iomap->dax_dev, pgoff, PHYS_PFN(PAGE_SIZE),
- &kaddr, &pfn);
- if (rc < 0) {
- dax_read_unlock(id);
- return rc;
+ length = dax_direct_access(iomap->dax_dev, pgoff, PHYS_PFN(size),
+ &kaddr, pfnp);
+ if (length < 0) {
+ rc = length;
+ goto out;
}
+ rc = -EINVAL;
+ if (PFN_PHYS(length) < size)
+ goto out;
+ if (pfn_t_to_pfn(*pfnp) & (PHYS_PFN(size)-1))
+ goto out;
+ /* For larger pages we need devmap */
+ if (length > 1 && !pfn_t_devmap(*pfnp))
+ goto out;
+ rc = 0;
+out:
dax_read_unlock(id);
+ return rc;
+}
+
+static int dax_insert_mapping(struct vm_fault *vmf, struct iomap *iomap,
+ loff_t pos, void *entry)
+{
+ const sector_t sector = dax_iomap_sector(iomap, pos);
+ struct vm_area_struct *vma = vmf->vma;
+ struct address_space *mapping = vma->vm_file->f_mapping;
+ unsigned long vaddr = vmf->address;
+ void *ret;
+ int rc;
+ pfn_t pfn;
+
+ rc = dax_iomap_pfn(iomap, pos, PAGE_SIZE, &pfn);
+ if (rc < 0)
+ return rc;
ret = dax_insert_mapping_entry(mapping, vmf, entry, sector, 0);
if (IS_ERR(ret))
@@ -1223,46 +1246,26 @@ static int dax_pmd_insert_mapping(struct vm_fault *vmf, struct iomap *iomap,
{
struct address_space *mapping = vmf->vma->vm_file->f_mapping;
const sector_t sector = dax_iomap_sector(iomap, pos);
- struct dax_device *dax_dev = iomap->dax_dev;
- struct block_device *bdev = iomap->bdev;
struct inode *inode = mapping->host;
- const size_t size = PMD_SIZE;
- void *ret = NULL, *kaddr;
- long length = 0;
- pgoff_t pgoff;
+ void *ret = NULL;
pfn_t pfn = {};
- int id;
+ int rc;
- if (bdev_dax_pgoff(bdev, sector, size, &pgoff) != 0)
+ rc = dax_iomap_pfn(iomap, pos, PMD_SIZE, &pfn);
+ if (rc < 0)
goto fallback;
- id = dax_read_lock();
- length = dax_direct_access(dax_dev, pgoff, PHYS_PFN(size), &kaddr, &pfn);
- if (length < 0)
- goto unlock_fallback;
- length = PFN_PHYS(length);
-
- if (length < size)
- goto unlock_fallback;
- if (pfn_t_to_pfn(pfn) & PG_PMD_COLOUR)
- goto unlock_fallback;
- if (!pfn_t_devmap(pfn))
- goto unlock_fallback;
- dax_read_unlock(id);
-
ret = dax_insert_mapping_entry(mapping, vmf, entry, sector,
RADIX_DAX_PMD);
if (IS_ERR(ret))
goto fallback;
- trace_dax_pmd_insert_mapping(inode, vmf, length, pfn, ret);
+ trace_dax_pmd_insert_mapping(inode, vmf, PMD_SIZE, pfn, ret);
return vmf_insert_pfn_pmd(vmf->vma, vmf->address, vmf->pmd,
pfn, vmf->flags & FAULT_FLAG_WRITE);
-unlock_fallback:
- dax_read_unlock(id);
fallback:
- trace_dax_pmd_insert_mapping_fallback(inode, vmf, length, pfn, ret);
+ trace_dax_pmd_insert_mapping_fallback(inode, vmf, PMD_SIZE, pfn, ret);
return VM_FAULT_FALLBACK;
}
--
2.12.3
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
dax_pmd_insert_mapping() has only one callsite and we will need to
further fine tune what it does for synchronous faults. Just inline it
into the callsite so that we don't have to pass awkward bools around.
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Ross Zwisler <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
---
fs/dax.c | 47 +++++++++++++++++--------------------------
include/trace/events/fs_dax.h | 1 -
2 files changed, 19 insertions(+), 29 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index 5b20c6456926..675fab8ec41f 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1235,33 +1235,11 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf,
}
#ifdef CONFIG_FS_DAX_PMD
-static int dax_pmd_insert_mapping(struct vm_fault *vmf, struct iomap *iomap,
- loff_t pos, void *entry)
-{
- struct address_space *mapping = vmf->vma->vm_file->f_mapping;
- const sector_t sector = dax_iomap_sector(iomap, pos);
- struct inode *inode = mapping->host;
- void *ret = NULL;
- pfn_t pfn = {};
- int rc;
-
- rc = dax_iomap_pfn(iomap, pos, PMD_SIZE, &pfn);
- if (rc < 0)
- goto fallback;
-
- ret = dax_insert_mapping_entry(mapping, vmf, entry, sector,
- RADIX_DAX_PMD);
- if (IS_ERR(ret))
- goto fallback;
-
- trace_dax_pmd_insert_mapping(inode, vmf, PMD_SIZE, pfn, ret);
- return vmf_insert_pfn_pmd(vmf->vma, vmf->address, vmf->pmd,
- pfn, vmf->flags & FAULT_FLAG_WRITE);
-
-fallback:
- trace_dax_pmd_insert_mapping_fallback(inode, vmf, PMD_SIZE, pfn, ret);
- return VM_FAULT_FALLBACK;
-}
+/*
+ * The 'colour' (ie low bits) within a PMD of a page offset. This comes up
+ * more often than one might expect in the below functions.
+ */
+#define PG_PMD_COLOUR ((PMD_SIZE >> PAGE_SHIFT) - 1)
static int dax_pmd_load_hole(struct vm_fault *vmf, struct iomap *iomap,
void *entry)
@@ -1317,6 +1295,7 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
void *entry;
loff_t pos;
int error;
+ pfn_t pfn;
/*
* Check whether offset isn't beyond end of file now. Caller is
@@ -1394,7 +1373,19 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
switch (iomap.type) {
case IOMAP_MAPPED:
- result = dax_pmd_insert_mapping(vmf, &iomap, pos, entry);
+ error = dax_iomap_pfn(&iomap, pos, PMD_SIZE, &pfn);
+ if (error < 0)
+ goto finish_iomap;
+
+ entry = dax_insert_mapping_entry(mapping, vmf, entry,
+ dax_iomap_sector(&iomap, pos),
+ RADIX_DAX_PMD);
+ if (IS_ERR(entry))
+ goto finish_iomap;
+
+ trace_dax_pmd_insert_mapping(inode, vmf, PMD_SIZE, pfn, entry);
+ result = vmf_insert_pfn_pmd(vma, vmf->address, vmf->pmd, pfn,
+ write);
break;
case IOMAP_UNWRITTEN:
case IOMAP_HOLE:
diff --git a/include/trace/events/fs_dax.h b/include/trace/events/fs_dax.h
index fbc4a06f7310..88a9d19b8ff8 100644
--- a/include/trace/events/fs_dax.h
+++ b/include/trace/events/fs_dax.h
@@ -148,7 +148,6 @@ DEFINE_EVENT(dax_pmd_insert_mapping_class, name, \
TP_ARGS(inode, vmf, length, pfn, radix_entry))
DEFINE_PMD_INSERT_MAPPING_EVENT(dax_pmd_insert_mapping);
-DEFINE_PMD_INSERT_MAPPING_EVENT(dax_pmd_insert_mapping_fallback);
DECLARE_EVENT_CLASS(dax_pte_fault_class,
TP_PROTO(struct inode *inode, struct vm_fault *vmf, int result),
--
2.12.3
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>