2021-04-27 22:54:43

by Axel Rasmussen

[permalink] [raw]
Subject: [PATCH v5 00/10] userfaultfd: add minor fault handling for shmem

Base
====

This series is based on (and therefore should apply cleanly to) the tag
"v5.12-rc7-mmots-2021-04-11-20-49", additionally with Peter's selftest cleanup
series applied first:

https://lore.kernel.org/patchwork/cover/1412450/

Changelog
=========

v4->v5:
- Picked up {Reviewed,Acked}-by's.
- Fix cleanup in error path in shmem_mcopy_atomic_pte(). [Hugh, Peter]
- Mention switching to lru_cache_add() in the commit message of 9/10. [Hugh]
- Split + reorder commits, so now we 1) implement the faulting path, 2)
implement the CONTINUE ioctl, and 3) advertise the feature. Squash the
documentation update into step (3). [Hugh, Peter]
- Reorder install_pte() cleanup to come before selftest changes. [Hugh]

v3->v4:
- Fix handling of the shmem private mcopy case. Previously, I had (incorrectly)
assumed that !vma_is_anonymous() was equivalent to "the page will be in the
page cache". But, in this case we have an optimization where we allocate a new
*anonymous* page. So, use a new "bool page_in_cache" instead, which checks if
page->mapping is set. Correct several places with this new check. [Hugh]
- Fix calling mm_counter() before page_add_..._rmap(). [Hugh]
- When modifying shmem_mcopy_atomic_pte() to use the new install_pte() helper,
just use lru_cache_add_inactive_or_unevictable(), no need to branch and maybe
use lru_cache_add(). [Hugh]
- De-pluralize mcopy_atomic_install_pte(s). [Hugh]
- Make "writable" a bool, and initialize consistently. [Hugh]

v2->v3:
- Picked up {Reviewed,Acked}-by's.
- Reorder commits: introduce CONTINUE before MINOR registration. [Hugh, Peter]
- Don't try to {unlock,put}_page an xarray value in shmem_getpage_gfp. [Hugh]
- Move enum mcopy_atomic_mode forward declare out of CONFIG_HUGETLB_PAGE. [Hugh]
- Keep mistakenly removed UFFD_USER_MODE_ONLY in selftest. [Peter]
- Cleanup context management in self test (make clear implicit, remove unneeded
return values now that we have err()). [Peter]
- Correct dst_pte argument to dst_pmd in shmem_mcopy_atomic_pte macro. [Hugh]
- Mention the new shmem support feature in documentation. [Hugh]

v1->v2:
- Pick up Reviewed-by's.
- Don't swapin page when a minor fault occurs. Notice that it needs to be
swapped in, and just immediately fire the minor fault. Let a future CONTINUE
deal with swapping in the page. [Peter]
- Clarify comment about i_size checks in mm/userfaultfd.c. [Peter]
- Only forward declare once (out of #ifdef) in hugetlb.h. [Peter]

Changes since [2]:
- Squash the fixes ([2]) in with the original series ([1]). This makes reviewing
easier, as we no longer have to sift through deltas undoing what we had done
before. [Hugh, Peter]
- Modify shmem_mcopy_atomic_pte() to use the new mcopy_atomic_install_ptes()
helper, reducing code duplication. [Hugh]
- Properly trigger handle_userfault() in the shmem_swapin_page() case. [Hugh]
- Use shmem_getpage() instead of find_lock_page() to lookup the existing page in
for continue. This properly deals with swapped-out pages. [Hugh]
- Unconditionally pte_mkdirty() for anon memory (as before). [Peter]
- Don't include userfaultfd_k.h in either hugetlb.h or shmem_fs.h. [Hugh]
- Add comment for UFFD_FEATURE_MINOR_SHMEM (to match _HUGETLBFS). [Hugh]
- Fix some small cleanup issues (parens, reworded conditionals, reduced plumbing
of some parameters, simplify labels/gotos, ...). [Hugh, Peter]

Overview
========

See the series which added minor faults for hugetlbfs [3] for a detailed
overview of minor fault handling in general. This series adds the same support
for shmem-backed areas.

This series is structured as follows:

- Commits 1 and 2 are cleanups.
- Commits 3 and 4 implement the new feature (minor fault handling for shmem).
- Commit 5 advertises that the feature is now available since at this point it's
fully implemented.
- Commit 6 is a final cleanup, modifying an existing code path to re-use a new
helper we've introduced.
- Commits 7, 8, 9, 10 update the userfaultfd selftest to exercise the feature.

Use Case
========

In some cases it is useful to have VM memory backed by tmpfs instead of
hugetlbfs. So, this feature will be used to support the same VM live migration
use case described in my original series.

Additionally, Android folks (Lokesh Gidra <[email protected]>) hope to
optimize the Android Runtime garbage collector using this feature:

"The plan is to use userfaultfd for concurrently compacting the heap. With
this feature, the heap can be shared-mapped at another location where the
GC-thread(s) could continue the compaction operation without the need to
invoke userfault ioctl(UFFDIO_COPY) each time. OTOH, if and when Java threads
get faults on the heap, UFFDIO_CONTINUE can be used to resume execution.
Furthermore, this feature enables updating references in the 'non-moving'
portion of the heap efficiently. Without this feature, uneccessary page
copying (ioctl(UFFDIO_COPY)) would be required."

[1] https://lore.kernel.org/patchwork/cover/1388144/
[2] https://lore.kernel.org/patchwork/patch/1408161/
[3] https://lore.kernel.org/linux-fsdevel/[email protected]/T/#t

Axel Rasmussen (10):
userfaultfd/hugetlbfs: avoid including userfaultfd_k.h in hugetlb.h
userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte
userfaultfd/shmem: support minor fault registration for shmem
userfaultfd/shmem: support UFFDIO_CONTINUE for shmem
userfaultfd/shmem: advertise shmem minor fault support
userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()
userfaultfd/selftests: use memfd_create for shmem test type
userfaultfd/selftests: create alias mappings in the shmem test
userfaultfd/selftests: reinitialize test context in each test
userfaultfd/selftests: exercise minor fault handling shmem support

Documentation/admin-guide/mm/userfaultfd.rst | 3 +-
fs/userfaultfd.c | 6 +-
include/linux/hugetlb.h | 4 +-
include/linux/shmem_fs.h | 17 +-
include/linux/userfaultfd_k.h | 5 +
include/uapi/linux/userfaultfd.h | 7 +-
mm/hugetlb.c | 1 +
mm/memory.c | 8 +-
mm/shmem.c | 110 +++-----
mm/userfaultfd.c | 175 ++++++++----
tools/testing/selftests/vm/userfaultfd.c | 274 ++++++++++++-------
11 files changed, 360 insertions(+), 250 deletions(-)

--
2.31.1.498.g6c1eba8ee3d-goog


2021-04-27 22:55:26

by Axel Rasmussen

[permalink] [raw]
Subject: [PATCH v5 09/10] userfaultfd/selftests: reinitialize test context in each test

Currently, the context (fds, mmap-ed areas, etc.) are global. Each test
mutates this state in some way, in some cases really "clobbering it"
(e.g., the events test mremap-ing area_dst over the top of area_src, or
the minor faults tests overwriting the count_verify values in the test
areas). We run the tests in a particular order, each test is careful to
make the right assumptions about its starting state, etc.

But, this is fragile. It's better for a test's success or failure to not
depend on what some other prior test case did to the global state.

To that end, clear and reinitialize the test context at the start of
each test case, so whatever prior test cases did doesn't affect future
tests.

This is particularly relevant to this series because the events test's
mremap of area_dst screws up assumptions the minor fault test was
relying on. This wasn't a problem for hugetlb, as we don't mremap in
that case.

Signed-off-by: Axel Rasmussen <[email protected]>
---
tools/testing/selftests/vm/userfaultfd.c | 215 ++++++++++++-----------
1 file changed, 116 insertions(+), 99 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 1f65c4ab7994..3fbc69f513dc 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -89,7 +89,8 @@ static int shm_fd;
static int huge_fd;
static char *huge_fd_off0;
static unsigned long long *count_verify;
-static int uffd, uffd_flags, finished, *pipefd;
+static int uffd = -1;
+static int uffd_flags, finished, *pipefd;
static char *area_src, *area_src_alias, *area_dst, *area_dst_alias;
static char *zeropage;
pthread_attr_t attr;
@@ -342,6 +343,111 @@ static struct uffd_test_ops hugetlb_uffd_test_ops = {

static struct uffd_test_ops *uffd_test_ops;

+static void userfaultfd_open(uint64_t *features)
+{
+ struct uffdio_api uffdio_api;
+
+ uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY);
+ if (uffd < 0)
+ err("userfaultfd syscall not available in this kernel");
+ uffd_flags = fcntl(uffd, F_GETFD, NULL);
+
+ uffdio_api.api = UFFD_API;
+ uffdio_api.features = *features;
+ if (ioctl(uffd, UFFDIO_API, &uffdio_api))
+ err("UFFDIO_API failed.\nPlease make sure to "
+ "run with either root or ptrace capability.");
+ if (uffdio_api.api != UFFD_API)
+ err("UFFDIO_API error: %" PRIu64, (uint64_t)uffdio_api.api);
+
+ *features = uffdio_api.features;
+}
+
+static inline void munmap_area(void **area)
+{
+ if (*area)
+ if (munmap(*area, nr_pages * page_size))
+ err("munmap");
+
+ *area = NULL;
+}
+
+static void uffd_test_ctx_clear(void)
+{
+ size_t i;
+
+ if (pipefd) {
+ for (i = 0; i < nr_cpus * 2; ++i) {
+ if (close(pipefd[i]))
+ err("close pipefd");
+ }
+ free(pipefd);
+ pipefd = NULL;
+ }
+
+ if (count_verify) {
+ free(count_verify);
+ count_verify = NULL;
+ }
+
+ if (uffd != -1) {
+ if (close(uffd))
+ err("close uffd");
+ uffd = -1;
+ }
+
+ huge_fd_off0 = NULL;
+ munmap_area((void **)&area_src);
+ munmap_area((void **)&area_src_alias);
+ munmap_area((void **)&area_dst);
+ munmap_area((void **)&area_dst_alias);
+}
+
+static void uffd_test_ctx_init_ext(uint64_t *features)
+{
+ unsigned long nr, cpu;
+
+ uffd_test_ctx_clear();
+
+ uffd_test_ops->allocate_area((void **)&area_src);
+ uffd_test_ops->allocate_area((void **)&area_dst);
+
+ uffd_test_ops->release_pages(area_src);
+ uffd_test_ops->release_pages(area_dst);
+
+ userfaultfd_open(features);
+
+ count_verify = malloc(nr_pages * sizeof(unsigned long long));
+ if (!count_verify)
+ err("count_verify");
+
+ for (nr = 0; nr < nr_pages; nr++) {
+ *area_mutex(area_src, nr) =
+ (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER;
+ count_verify[nr] = *area_count(area_src, nr) = 1;
+ /*
+ * In the transition between 255 to 256, powerpc will
+ * read out of order in my_bcmp and see both bytes as
+ * zero, so leave a placeholder below always non-zero
+ * after the count, to avoid my_bcmp to trigger false
+ * positives.
+ */
+ *(area_count(area_src, nr) + 1) = 1;
+ }
+
+ pipefd = malloc(sizeof(int) * nr_cpus * 2);
+ if (!pipefd)
+ err("pipefd");
+ for (cpu = 0; cpu < nr_cpus; cpu++)
+ if (pipe2(&pipefd[cpu * 2], O_CLOEXEC | O_NONBLOCK))
+ err("pipe");
+}
+
+static inline void uffd_test_ctx_init(uint64_t features)
+{
+ uffd_test_ctx_init_ext(&features);
+}
+
static int my_bcmp(char *str1, char *str2, size_t n)
{
unsigned long i;
@@ -726,40 +832,6 @@ static int stress(struct uffd_stats *uffd_stats)
return 0;
}

-static int userfaultfd_open_ext(uint64_t *features)
-{
- struct uffdio_api uffdio_api;
-
- uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY);
- if (uffd < 0) {
- fprintf(stderr,
- "userfaultfd syscall not available in this kernel\n");
- return 1;
- }
- uffd_flags = fcntl(uffd, F_GETFD, NULL);
-
- uffdio_api.api = UFFD_API;
- uffdio_api.features = *features;
- if (ioctl(uffd, UFFDIO_API, &uffdio_api)) {
- fprintf(stderr, "UFFDIO_API failed.\nPlease make sure to "
- "run with either root or ptrace capability.\n");
- return 1;
- }
- if (uffdio_api.api != UFFD_API) {
- fprintf(stderr, "UFFDIO_API error: %" PRIu64 "\n",
- (uint64_t)uffdio_api.api);
- return 1;
- }
-
- *features = uffdio_api.features;
- return 0;
-}
-
-static int userfaultfd_open(uint64_t features)
-{
- return userfaultfd_open_ext(&features);
-}
-
sigjmp_buf jbuf, *sigbuf;

static void sighndl(int sig, siginfo_t *siginfo, void *ptr)
@@ -868,6 +940,8 @@ static int faulting_process(int signal_test)
MREMAP_MAYMOVE | MREMAP_FIXED, area_src);
if (area_dst == MAP_FAILED)
err("mremap");
+ /* Reset area_src since we just clobbered it */
+ area_src = NULL;

for (; nr < nr_pages; nr++) {
count = *area_count(area_dst, nr);
@@ -961,10 +1035,8 @@ static int userfaultfd_zeropage_test(void)
printf("testing UFFDIO_ZEROPAGE: ");
fflush(stdout);

- uffd_test_ops->release_pages(area_dst);
+ uffd_test_ctx_init(0);

- if (userfaultfd_open(0))
- return 1;
uffdio_register.range.start = (unsigned long) area_dst;
uffdio_register.range.len = nr_pages * page_size;
uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
@@ -981,7 +1053,6 @@ static int userfaultfd_zeropage_test(void)
if (my_bcmp(area_dst, zeropage, page_size))
err("zeropage is not zero");

- close(uffd);
printf("done.\n");
return 0;
}
@@ -999,12 +1070,10 @@ static int userfaultfd_events_test(void)
printf("testing events (fork, remap, remove): ");
fflush(stdout);

- uffd_test_ops->release_pages(area_dst);
-
features = UFFD_FEATURE_EVENT_FORK | UFFD_FEATURE_EVENT_REMAP |
UFFD_FEATURE_EVENT_REMOVE;
- if (userfaultfd_open(features))
- return 1;
+ uffd_test_ctx_init(features);
+
fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);

uffdio_register.range.start = (unsigned long) area_dst;
@@ -1037,8 +1106,6 @@ static int userfaultfd_events_test(void)
if (pthread_join(uffd_mon, NULL))
return 1;

- close(uffd);
-
uffd_stats_report(&stats, 1);

return stats.missing_faults != nr_pages;
@@ -1058,11 +1125,9 @@ static int userfaultfd_sig_test(void)
printf("testing signal delivery: ");
fflush(stdout);

- uffd_test_ops->release_pages(area_dst);
-
features = UFFD_FEATURE_EVENT_FORK|UFFD_FEATURE_SIGBUS;
- if (userfaultfd_open(features))
- return 1;
+ uffd_test_ctx_init(features);
+
fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);

uffdio_register.range.start = (unsigned long) area_dst;
@@ -1103,7 +1168,6 @@ static int userfaultfd_sig_test(void)
printf("done.\n");
if (userfaults)
err("Signal test failed, userfaults: %ld", userfaults);
- close(uffd);

return userfaults != 0;
}
@@ -1126,10 +1190,7 @@ static int userfaultfd_minor_test(void)
printf("testing minor faults: ");
fflush(stdout);

- uffd_test_ops->release_pages(area_dst);
-
- if (userfaultfd_open_ext(&features))
- return 1;
+ uffd_test_ctx_init_ext(&features);
/* If kernel reports the feature isn't supported, skip the test. */
if (!(features & UFFD_FEATURE_MINOR_HUGETLBFS)) {
printf("skipping test due to lack of feature support\n");
@@ -1183,8 +1244,6 @@ static int userfaultfd_minor_test(void)
if (pthread_join(uffd_mon, NULL))
return 1;

- close(uffd);
-
uffd_stats_report(&stats, 1);

return stats.missing_faults != 0 || stats.minor_faults != nr_pages;
@@ -1196,50 +1255,9 @@ static int userfaultfd_stress(void)
char *tmp_area;
unsigned long nr;
struct uffdio_register uffdio_register;
- unsigned long cpu;
struct uffd_stats uffd_stats[nr_cpus];

- uffd_test_ops->allocate_area((void **)&area_src);
- if (!area_src)
- return 1;
- uffd_test_ops->allocate_area((void **)&area_dst);
- if (!area_dst)
- return 1;
-
- if (userfaultfd_open(0))
- return 1;
-
- count_verify = malloc(nr_pages * sizeof(unsigned long long));
- if (!count_verify) {
- perror("count_verify");
- return 1;
- }
-
- for (nr = 0; nr < nr_pages; nr++) {
- *area_mutex(area_src, nr) = (pthread_mutex_t)
- PTHREAD_MUTEX_INITIALIZER;
- count_verify[nr] = *area_count(area_src, nr) = 1;
- /*
- * In the transition between 255 to 256, powerpc will
- * read out of order in my_bcmp and see both bytes as
- * zero, so leave a placeholder below always non-zero
- * after the count, to avoid my_bcmp to trigger false
- * positives.
- */
- *(area_count(area_src, nr) + 1) = 1;
- }
-
- pipefd = malloc(sizeof(int) * nr_cpus * 2);
- if (!pipefd) {
- perror("pipefd");
- return 1;
- }
- for (cpu = 0; cpu < nr_cpus; cpu++) {
- if (pipe2(&pipefd[cpu*2], O_CLOEXEC | O_NONBLOCK)) {
- perror("pipe");
- return 1;
- }
- }
+ uffd_test_ctx_init(0);

if (posix_memalign(&area, page_size, page_size))
err("out of memory");
@@ -1360,7 +1378,6 @@ static int userfaultfd_stress(void)
uffd_stats_report(uffd_stats, nr_cpus);
}

- close(uffd);
return userfaultfd_zeropage_test() || userfaultfd_sig_test()
|| userfaultfd_events_test() || userfaultfd_minor_test();
}
--
2.31.1.498.g6c1eba8ee3d-goog

2021-04-27 22:56:17

by Axel Rasmussen

[permalink] [raw]
Subject: [PATCH v5 05/10] userfaultfd/shmem: advertise shmem minor fault support

Now that the feature is fully implemented (the faulting path hooks exist
so userspace is notified, and the ioctl to resolve such faults is
available), advertise this as a supported feature.

Signed-off-by: Axel Rasmussen <[email protected]>
---
Documentation/admin-guide/mm/userfaultfd.rst | 3 ++-
fs/userfaultfd.c | 3 ++-
include/uapi/linux/userfaultfd.h | 7 ++++++-
3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 3aa38e8b8361..6528036093e1 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -77,7 +77,8 @@ events, except page fault notifications, may be generated:

- ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports
``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory
- areas.
+ areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
+ support for shmem virtual memory areas.

The userland application should set the feature flags it intends to use
when invoking the ``UFFDIO_API`` ioctl, to request that those features be
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 468556fb04a9..9f3b8684cf3c 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1940,7 +1940,8 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
/* report all available features and ioctls to userland */
uffdio_api.features = UFFD_API_FEATURES;
#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
- uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS;
+ uffdio_api.features &=
+ ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
#endif
uffdio_api.ioctls = UFFD_API_IOCTLS;
ret = -EFAULT;
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index bafbeb1a2624..159a74e9564f 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -31,7 +31,8 @@
UFFD_FEATURE_MISSING_SHMEM | \
UFFD_FEATURE_SIGBUS | \
UFFD_FEATURE_THREAD_ID | \
- UFFD_FEATURE_MINOR_HUGETLBFS)
+ UFFD_FEATURE_MINOR_HUGETLBFS | \
+ UFFD_FEATURE_MINOR_SHMEM)
#define UFFD_API_IOCTLS \
((__u64)1 << _UFFDIO_REGISTER | \
(__u64)1 << _UFFDIO_UNREGISTER | \
@@ -185,6 +186,9 @@ struct uffdio_api {
* UFFD_FEATURE_MINOR_HUGETLBFS indicates that minor faults
* can be intercepted (via REGISTER_MODE_MINOR) for
* hugetlbfs-backed pages.
+ *
+ * UFFD_FEATURE_MINOR_SHMEM indicates the same support as
+ * UFFD_FEATURE_MINOR_HUGETLBFS, but for shmem-backed pages instead.
*/
#define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0)
#define UFFD_FEATURE_EVENT_FORK (1<<1)
@@ -196,6 +200,7 @@ struct uffdio_api {
#define UFFD_FEATURE_SIGBUS (1<<7)
#define UFFD_FEATURE_THREAD_ID (1<<8)
#define UFFD_FEATURE_MINOR_HUGETLBFS (1<<9)
+#define UFFD_FEATURE_MINOR_SHMEM (1<<10)
__u64 features;

__u64 ioctls;
--
2.31.1.498.g6c1eba8ee3d-goog

2021-04-27 22:56:41

by Axel Rasmussen

[permalink] [raw]
Subject: [PATCH v5 08/10] userfaultfd/selftests: create alias mappings in the shmem test

Previously, we just allocated two shm areas: area_src and area_dst. With
this commit, change this so we also allocate area_src_alias, and
area_dst_alias.

area_*_alias and area_* (respectively) point to the same underlying
physical pages, but are different VMAs. In a future commit in this
series, we'll leverage this setup to exercise minor fault handling
support for shmem, just like we do in the hugetlb_shared test.

Reviewed-by: Peter Xu <[email protected]>
Signed-off-by: Axel Rasmussen <[email protected]>
---
tools/testing/selftests/vm/userfaultfd.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index fc40831f818f..1f65c4ab7994 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -278,13 +278,29 @@ static void shmem_release_pages(char *rel_area)

static void shmem_allocate_area(void **alloc_area)
{
- unsigned long offset =
- alloc_area == (void **)&area_src ? 0 : nr_pages * page_size;
+ void *area_alias = NULL;
+ bool is_src = alloc_area == (void **)&area_src;
+ unsigned long offset = is_src ? 0 : nr_pages * page_size;

*alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
MAP_SHARED, shm_fd, offset);
if (*alloc_area == MAP_FAILED)
err("mmap of memfd failed");
+
+ area_alias = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
+ MAP_SHARED, shm_fd, offset);
+ if (area_alias == MAP_FAILED)
+ err("mmap of memfd alias failed");
+
+ if (is_src)
+ area_src_alias = area_alias;
+ else
+ area_dst_alias = area_alias;
+}
+
+static void shmem_alias_mapping(__u64 *start, size_t len, unsigned long offset)
+{
+ *start = (unsigned long)area_dst_alias + offset;
}

struct uffd_test_ops {
@@ -314,7 +330,7 @@ static struct uffd_test_ops shmem_uffd_test_ops = {
.expected_ioctls = SHMEM_EXPECTED_IOCTLS,
.allocate_area = shmem_allocate_area,
.release_pages = shmem_release_pages,
- .alias_mapping = noop_alias_mapping,
+ .alias_mapping = shmem_alias_mapping,
};

static struct uffd_test_ops hugetlb_uffd_test_ops = {
--
2.31.1.498.g6c1eba8ee3d-goog

2021-04-27 22:56:51

by Axel Rasmussen

[permalink] [raw]
Subject: [PATCH v5 07/10] userfaultfd/selftests: use memfd_create for shmem test type

This is a preparatory commit. In the future, we want to be able to setup
alias mappings for area_src and area_dst in the shmem test, like we do
in the hugetlb_shared test. With a VMA obtained via
mmap(MAP_ANONYMOUS | MAP_SHARED), it isn't clear how to do this.

So, mmap() with an fd, so we can create alias mappings. Use memfd_create
instead of actually passing in a tmpfs path like hugetlb does, since
it's more convenient / simpler to run, and works just as well.

Future commits will:

1. Setup the alias mappings.
2. Extend our tests to actually take advantage of this, to test new
userfaultfd behavior being introduced in this series.

Also, a small fix in the area we're changing: when the hugetlb setup
fails in main(), pass in the right argv[] so we actually print out the
hugetlb file path.

Reviewed-by: Peter Xu <[email protected]>
Signed-off-by: Axel Rasmussen <[email protected]>
---
tools/testing/selftests/vm/userfaultfd.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 6339aeaeeff8..fc40831f818f 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -85,6 +85,7 @@ static bool test_uffdio_wp = false;
static bool test_uffdio_minor = false;

static bool map_shared;
+static int shm_fd;
static int huge_fd;
static char *huge_fd_off0;
static unsigned long long *count_verify;
@@ -277,8 +278,11 @@ static void shmem_release_pages(char *rel_area)

static void shmem_allocate_area(void **alloc_area)
{
+ unsigned long offset =
+ alloc_area == (void **)&area_src ? 0 : nr_pages * page_size;
+
*alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
- MAP_ANONYMOUS | MAP_SHARED, -1, 0);
+ MAP_SHARED, shm_fd, offset);
if (*alloc_area == MAP_FAILED)
err("mmap of memfd failed");
}
@@ -1448,6 +1452,16 @@ int main(int argc, char **argv)
err("Open of %s failed", argv[4]);
if (ftruncate(huge_fd, 0))
err("ftruncate %s to size 0 failed", argv[4]);
+ } else if (test_type == TEST_SHMEM) {
+ shm_fd = memfd_create(argv[0], 0);
+ if (shm_fd < 0)
+ err("memfd_create");
+ if (ftruncate(shm_fd, nr_pages * page_size * 2))
+ err("ftruncate");
+ if (fallocate(shm_fd,
+ FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, 0,
+ nr_pages * page_size * 2))
+ err("fallocate");
}
printf("nr_pages: %lu, nr_pages_per_cpu: %lu\n",
nr_pages, nr_pages_per_cpu);
--
2.31.1.498.g6c1eba8ee3d-goog

2021-04-27 22:57:22

by Axel Rasmussen

[permalink] [raw]
Subject: [PATCH v5 10/10] userfaultfd/selftests: exercise minor fault handling shmem support

Enable test_uffdio_minor for test_type == TEST_SHMEM, and modify the
test slightly to pass in / check for the right feature flags.

Signed-off-by: Axel Rasmussen <[email protected]>
---
tools/testing/selftests/vm/userfaultfd.c | 29 ++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 3fbc69f513dc..a7ecc9993439 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -474,6 +474,7 @@ static void wp_range(int ufd, __u64 start, __u64 len, bool wp)
static void continue_range(int ufd, __u64 start, __u64 len)
{
struct uffdio_continue req;
+ int ret;

req.range.start = start;
req.range.len = len;
@@ -482,6 +483,17 @@ static void continue_range(int ufd, __u64 start, __u64 len)
if (ioctl(ufd, UFFDIO_CONTINUE, &req))
err("UFFDIO_CONTINUE failed for address 0x%" PRIx64,
(uint64_t)start);
+
+ /*
+ * Error handling within the kernel for continue is subtly different
+ * from copy or zeropage, so it may be a source of bugs. Trigger an
+ * error (-EEXIST) on purpose, to verify doing so doesn't cause a BUG.
+ */
+ req.mapped = 0;
+ ret = ioctl(ufd, UFFDIO_CONTINUE, &req);
+ if (ret >= 0 || req.mapped != -EEXIST)
+ err("failed to exercise UFFDIO_CONTINUE error handling, ret=%d, mapped=%" PRId64,
+ ret, (int64_t) req.mapped);
}

static void *locking_thread(void *arg)
@@ -1182,7 +1194,7 @@ static int userfaultfd_minor_test(void)
void *expected_page;
char c;
struct uffd_stats stats = { 0 };
- uint64_t features = UFFD_FEATURE_MINOR_HUGETLBFS;
+ uint64_t req_features, features_out;

if (!test_uffdio_minor)
return 0;
@@ -1190,9 +1202,17 @@ static int userfaultfd_minor_test(void)
printf("testing minor faults: ");
fflush(stdout);

- uffd_test_ctx_init_ext(&features);
- /* If kernel reports the feature isn't supported, skip the test. */
- if (!(features & UFFD_FEATURE_MINOR_HUGETLBFS)) {
+ if (test_type == TEST_HUGETLB)
+ req_features = UFFD_FEATURE_MINOR_HUGETLBFS;
+ else if (test_type == TEST_SHMEM)
+ req_features = UFFD_FEATURE_MINOR_SHMEM;
+ else
+ return 1;
+
+ features_out = req_features;
+ uffd_test_ctx_init_ext(&features_out);
+ /* If kernel reports required features aren't supported, skip test. */
+ if ((features_out & req_features) != req_features) {
printf("skipping test due to lack of feature support\n");
fflush(stdout);
return 0;
@@ -1426,6 +1446,7 @@ static void set_test_type(const char *type)
map_shared = true;
test_type = TEST_SHMEM;
uffd_test_ops = &shmem_uffd_test_ops;
+ test_uffdio_minor = true;
} else {
err("Unknown test type: %s", type);
}
--
2.31.1.498.g6c1eba8ee3d-goog

2021-04-27 22:57:32

by Axel Rasmussen

[permalink] [raw]
Subject: [PATCH v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()

In a previous commit, we added the mcopy_atomic_install_pte() helper.
This helper does the job of setting up PTEs for an existing page, to map
it into a given VMA. It deals with both the anon and shmem cases, as
well as the shared and private cases.

In other words, shmem_mcopy_atomic_pte() duplicates a case it already
handles. So, expose it, and let shmem_mcopy_atomic_pte() use it
directly, to reduce code duplication.

This requires that we refactor shmem_mcopy_atomic_pte() a bit:

Instead of doing accounting (shmem_recalc_inode() et al) part-way
through the PTE setup, do it afterward. This frees up
mcopy_atomic_install_pte() from having to care about this accounting,
and means we don't need to e.g. shmem_uncharge() in the error path.

A side effect is this switches shmem_mcopy_atomic_pte() to use
lru_cache_add_inactive_or_unevictable() instead of just lru_cache_add().
This wrapper does some extra accounting in an exceptional case, if
appropriate, so it's actually the more correct thing to use.

Signed-off-by: Axel Rasmussen <[email protected]>
---
include/linux/userfaultfd_k.h | 5 ++++
mm/shmem.c | 48 +++++------------------------------
mm/userfaultfd.c | 17 +++++--------
3 files changed, 18 insertions(+), 52 deletions(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 794d1538b8ba..39c094cc6641 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -53,6 +53,11 @@ enum mcopy_atomic_mode {
MCOPY_ATOMIC_CONTINUE,
};

+extern int mcopy_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
+ struct vm_area_struct *dst_vma,
+ unsigned long dst_addr, struct page *page,
+ bool newly_allocated, bool wp_copy);
+
extern ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
unsigned long src_start, unsigned long len,
bool *mmap_changing, __u64 mode);
diff --git a/mm/shmem.c b/mm/shmem.c
index 30c0bb501dc9..37db52f45cb5 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2378,10 +2378,8 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
struct address_space *mapping = inode->i_mapping;
gfp_t gfp = mapping_gfp_mask(mapping);
pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
- spinlock_t *ptl;
void *page_kaddr;
struct page *page;
- pte_t _dst_pte, *dst_pte;
int ret;
pgoff_t max_off;

@@ -2404,9 +2402,9 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
/* fallback to copy_from_user outside mmap_lock */
if (unlikely(ret)) {
*pagep = page;
- shmem_inode_unacct_blocks(inode, 1);
+ ret = -ENOENT;
/* don't free the page */
- return -ENOENT;
+ goto out_unacct_blocks;
}
} else { /* ZEROPAGE */
clear_highpage(page);
@@ -2432,32 +2430,10 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
if (ret)
goto out_release;

- _dst_pte = mk_pte(page, dst_vma->vm_page_prot);
- if (dst_vma->vm_flags & VM_WRITE)
- _dst_pte = pte_mkwrite(pte_mkdirty(_dst_pte));
- else {
- /*
- * We don't set the pte dirty if the vma has no
- * VM_WRITE permission, so mark the page dirty or it
- * could be freed from under us. We could do it
- * unconditionally before unlock_page(), but doing it
- * only if VM_WRITE is not set is faster.
- */
- set_page_dirty(page);
- }
-
- dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
-
- ret = -EFAULT;
- max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
- if (unlikely(pgoff >= max_off))
- goto out_release_unlock;
-
- ret = -EEXIST;
- if (!pte_none(*dst_pte))
- goto out_release_unlock;
-
- lru_cache_add(page);
+ ret = mcopy_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
+ page, true, false);
+ if (ret)
+ goto out_release;

spin_lock_irq(&info->lock);
info->alloced++;
@@ -2465,21 +2441,11 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
shmem_recalc_inode(inode);
spin_unlock_irq(&info->lock);

- inc_mm_counter(dst_mm, mm_counter_file(page));
- page_add_file_rmap(page, false);
- set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
-
- /* No need to invalidate - it was non-present before */
- update_mmu_cache(dst_vma, dst_addr, dst_pte);
- pte_unmap_unlock(dst_pte, ptl);
+ SetPageDirty(page);
unlock_page(page);
ret = 0;
out:
return ret;
-out_release_unlock:
- pte_unmap_unlock(dst_pte, ptl);
- ClearPageDirty(page);
- delete_from_page_cache(page);
out_release:
unlock_page(page);
put_page(page);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 51d8c0127161..3a9ddbb2dbbd 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -51,18 +51,13 @@ struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
/*
* Install PTEs, to map dst_addr (within dst_vma) to page.
*
- * This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),
- * whether or not dst_vma is VM_SHARED. It also handles the more general
- * MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file
- * backed, or not).
- *
- * Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by
- * shmem_mcopy_atomic_pte instead.
+ * This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
+ * and anon, and for both shared and private VMAs.
*/
-static int mcopy_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
- struct vm_area_struct *dst_vma,
- unsigned long dst_addr, struct page *page,
- bool newly_allocated, bool wp_copy)
+int mcopy_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
+ struct vm_area_struct *dst_vma,
+ unsigned long dst_addr, struct page *page,
+ bool newly_allocated, bool wp_copy)
{
int ret;
pte_t _dst_pte, *dst_pte;
--
2.31.1.498.g6c1eba8ee3d-goog

2021-04-28 00:06:06

by Hugh Dickins

[permalink] [raw]
Subject: Re: [PATCH v5 05/10] userfaultfd/shmem: advertise shmem minor fault support

On Tue, 27 Apr 2021, Axel Rasmussen wrote:

> Now that the feature is fully implemented (the faulting path hooks exist
> so userspace is notified, and the ioctl to resolve such faults is
> available), advertise this as a supported feature.
>
> Signed-off-by: Axel Rasmussen <[email protected]>

Acked-by: Hugh Dickins <[email protected]>

> ---
> Documentation/admin-guide/mm/userfaultfd.rst | 3 ++-
> fs/userfaultfd.c | 3 ++-
> include/uapi/linux/userfaultfd.h | 7 ++++++-
> 3 files changed, 10 insertions(+), 3 deletions(-)

2021-04-28 01:00:38

by Hugh Dickins

[permalink] [raw]
Subject: Re: [PATCH v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()

On Tue, 27 Apr 2021, Axel Rasmussen wrote:

> In a previous commit, we added the mcopy_atomic_install_pte() helper.
> This helper does the job of setting up PTEs for an existing page, to map
> it into a given VMA. It deals with both the anon and shmem cases, as
> well as the shared and private cases.
>
> In other words, shmem_mcopy_atomic_pte() duplicates a case it already
> handles. So, expose it, and let shmem_mcopy_atomic_pte() use it
> directly, to reduce code duplication.
>
> This requires that we refactor shmem_mcopy_atomic_pte() a bit:
>
> Instead of doing accounting (shmem_recalc_inode() et al) part-way
> through the PTE setup, do it afterward. This frees up
> mcopy_atomic_install_pte() from having to care about this accounting,
> and means we don't need to e.g. shmem_uncharge() in the error path.
>
> A side effect is this switches shmem_mcopy_atomic_pte() to use
> lru_cache_add_inactive_or_unevictable() instead of just lru_cache_add().
> This wrapper does some extra accounting in an exceptional case, if
> appropriate, so it's actually the more correct thing to use.
>
> Signed-off-by: Axel Rasmussen <[email protected]>

Not quite. Two things.

One, in this version, delete_from_page_cache(page) has vanished
from the particular error path which needs it.

Two, and I think this predates your changes (so needs a separate
fix patch first, for backport to stable? a user with bad intentions
might be able to trigger the BUG), in pondering the new error paths
and that /* don't free the page */ one in particular, isn't it the
case that the shmem_inode_acct_block() on entry might succeed the
first time, but atomic copy fail so -ENOENT, then something else
fill up the tmpfs before the retry comes in, so that retry then
fail with -ENOMEM, and hit the BUG_ON(page) in __mcopy_atomic()?

(As I understand it, the shmem_inode_unacct_blocks() has to be
done before returning, because the caller may be unable to retry.)

What the right fix is rather depends on other uses of __mcopy_atomic():
if they obviously cannot hit that BUG_ON(page), you may prefer to leave
it in, and fix it here where shmem_inode_acct_block() fails. Or you may
prefer instead to delete that "else BUG_ON(page);" - looks as if that
would end up doing the right thing. Peter may have a preference.

(Or, we could consider doing the shmem_inode_acct_block() only after
the page has been copied in: its current placing reflects how shmem.c
does it elsewhere, and there's reason for that, but it doesn't always
work out right. Don't be surprised if I change the ordering in future,
but it's probably best not to mess with that ordering now.)

Sorry, if this is a pre-existing issue, then we are taking advantage
of you, in asking you to fix it: but I hope that while you're in there,
it will make sense to do so.

Thanks,
Hugh

> ---
> include/linux/userfaultfd_k.h | 5 ++++
> mm/shmem.c | 48 +++++------------------------------
> mm/userfaultfd.c | 17 +++++--------
> 3 files changed, 18 insertions(+), 52 deletions(-)

2021-04-28 17:40:36

by Peter Xu

[permalink] [raw]
Subject: Re: [PATCH v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()

On Tue, Apr 27, 2021 at 05:58:16PM -0700, Hugh Dickins wrote:
> On Tue, 27 Apr 2021, Axel Rasmussen wrote:
>
> > In a previous commit, we added the mcopy_atomic_install_pte() helper.
> > This helper does the job of setting up PTEs for an existing page, to map
> > it into a given VMA. It deals with both the anon and shmem cases, as
> > well as the shared and private cases.
> >
> > In other words, shmem_mcopy_atomic_pte() duplicates a case it already
> > handles. So, expose it, and let shmem_mcopy_atomic_pte() use it
> > directly, to reduce code duplication.
> >
> > This requires that we refactor shmem_mcopy_atomic_pte() a bit:
> >
> > Instead of doing accounting (shmem_recalc_inode() et al) part-way
> > through the PTE setup, do it afterward. This frees up
> > mcopy_atomic_install_pte() from having to care about this accounting,
> > and means we don't need to e.g. shmem_uncharge() in the error path.
> >
> > A side effect is this switches shmem_mcopy_atomic_pte() to use
> > lru_cache_add_inactive_or_unevictable() instead of just lru_cache_add().
> > This wrapper does some extra accounting in an exceptional case, if
> > appropriate, so it's actually the more correct thing to use.
> >
> > Signed-off-by: Axel Rasmussen <[email protected]>
>
> Not quite. Two things.
>
> One, in this version, delete_from_page_cache(page) has vanished
> from the particular error path which needs it.

Agreed. I also spotted that the set_page_dirty() seems to have been overlooked
when reusing mcopy_atomic_install_pte(), which afaiu should be move into the
helper.

>
> Two, and I think this predates your changes (so needs a separate
> fix patch first, for backport to stable? a user with bad intentions
> might be able to trigger the BUG), in pondering the new error paths
> and that /* don't free the page */ one in particular, isn't it the
> case that the shmem_inode_acct_block() on entry might succeed the
> first time, but atomic copy fail so -ENOENT, then something else
> fill up the tmpfs before the retry comes in, so that retry then
> fail with -ENOMEM, and hit the BUG_ON(page) in __mcopy_atomic()?
>
> (As I understand it, the shmem_inode_unacct_blocks() has to be
> done before returning, because the caller may be unable to retry.)
>
> What the right fix is rather depends on other uses of __mcopy_atomic():
> if they obviously cannot hit that BUG_ON(page), you may prefer to leave
> it in, and fix it here where shmem_inode_acct_block() fails. Or you may
> prefer instead to delete that "else BUG_ON(page);" - looks as if that
> would end up doing the right thing. Peter may have a preference.

To me, the BUG_ON(page) wanted to guarantee mfill_atomic_pte() should have
consumed the page properly when possible. Removing the BUG_ON() looks good
already, it will just stop covering the case when e.g. ret==0.

So maybe slightly better to release the page when shmem_inode_acct_block()
fails (so as to still keep some guard on the page)?

Thanks,

--
Peter Xu

2021-04-28 17:40:39

by Axel Rasmussen

[permalink] [raw]
Subject: Re: [PATCH v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()

On Wed, Apr 28, 2021 at 8:56 AM Peter Xu <[email protected]> wrote:
>
> On Tue, Apr 27, 2021 at 05:58:16PM -0700, Hugh Dickins wrote:
> > On Tue, 27 Apr 2021, Axel Rasmussen wrote:
> >
> > > In a previous commit, we added the mcopy_atomic_install_pte() helper.
> > > This helper does the job of setting up PTEs for an existing page, to map
> > > it into a given VMA. It deals with both the anon and shmem cases, as
> > > well as the shared and private cases.
> > >
> > > In other words, shmem_mcopy_atomic_pte() duplicates a case it already
> > > handles. So, expose it, and let shmem_mcopy_atomic_pte() use it
> > > directly, to reduce code duplication.
> > >
> > > This requires that we refactor shmem_mcopy_atomic_pte() a bit:
> > >
> > > Instead of doing accounting (shmem_recalc_inode() et al) part-way
> > > through the PTE setup, do it afterward. This frees up
> > > mcopy_atomic_install_pte() from having to care about this accounting,
> > > and means we don't need to e.g. shmem_uncharge() in the error path.
> > >
> > > A side effect is this switches shmem_mcopy_atomic_pte() to use
> > > lru_cache_add_inactive_or_unevictable() instead of just lru_cache_add().
> > > This wrapper does some extra accounting in an exceptional case, if
> > > appropriate, so it's actually the more correct thing to use.
> > >
> > > Signed-off-by: Axel Rasmussen <[email protected]>
> >
> > Not quite. Two things.
> >
> > One, in this version, delete_from_page_cache(page) has vanished
> > from the particular error path which needs it.
>
> Agreed. I also spotted that the set_page_dirty() seems to have been overlooked
> when reusing mcopy_atomic_install_pte(), which afaiu should be move into the
> helper.

I think this is covered: we explicitly call SetPageDirty() just before
returning in shmem_mcopy_atomic_pte(). If I remember correctly from a
couple of revisions ago, we consciously put it here instead of in the
helper because it resulted in simpler code (error handling in
particular, I think?), and not all callers of the new helper need it.

>
> >
> > Two, and I think this predates your changes (so needs a separate
> > fix patch first, for backport to stable? a user with bad intentions
> > might be able to trigger the BUG), in pondering the new error paths
> > and that /* don't free the page */ one in particular, isn't it the
> > case that the shmem_inode_acct_block() on entry might succeed the
> > first time, but atomic copy fail so -ENOENT, then something else
> > fill up the tmpfs before the retry comes in, so that retry then
> > fail with -ENOMEM, and hit the BUG_ON(page) in __mcopy_atomic()?
> >
> > (As I understand it, the shmem_inode_unacct_blocks() has to be
> > done before returning, because the caller may be unable to retry.)
> >
> > What the right fix is rather depends on other uses of __mcopy_atomic():
> > if they obviously cannot hit that BUG_ON(page), you may prefer to leave
> > it in, and fix it here where shmem_inode_acct_block() fails. Or you may
> > prefer instead to delete that "else BUG_ON(page);" - looks as if that
> > would end up doing the right thing. Peter may have a preference.
>
> To me, the BUG_ON(page) wanted to guarantee mfill_atomic_pte() should have
> consumed the page properly when possible. Removing the BUG_ON() looks good
> already, it will just stop covering the case when e.g. ret==0.
>
> So maybe slightly better to release the page when shmem_inode_acct_block()
> fails (so as to still keep some guard on the page)?

This second issue, I will take some more time to investigate. :)

>
> Thanks,
>
> --
> Peter Xu
>

2021-04-28 19:22:03

by Peter Xu

[permalink] [raw]
Subject: Re: [PATCH v5 05/10] userfaultfd/shmem: advertise shmem minor fault support

On Tue, Apr 27, 2021 at 03:52:39PM -0700, Axel Rasmussen wrote:
> Now that the feature is fully implemented (the faulting path hooks exist
> so userspace is notified, and the ioctl to resolve such faults is
> available), advertise this as a supported feature.
>
> Signed-off-by: Axel Rasmussen <[email protected]>

Acked-by: Peter Xu <[email protected]>

--
Peter Xu

2021-04-28 20:50:27

by Peter Xu

[permalink] [raw]
Subject: Re: [PATCH v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()

On Wed, Apr 28, 2021 at 08:59:53AM -0700, Axel Rasmussen wrote:
> On Wed, Apr 28, 2021 at 8:56 AM Peter Xu <[email protected]> wrote:
> >
> > On Tue, Apr 27, 2021 at 05:58:16PM -0700, Hugh Dickins wrote:
> > > On Tue, 27 Apr 2021, Axel Rasmussen wrote:
> > >
> > > > In a previous commit, we added the mcopy_atomic_install_pte() helper.
> > > > This helper does the job of setting up PTEs for an existing page, to map
> > > > it into a given VMA. It deals with both the anon and shmem cases, as
> > > > well as the shared and private cases.
> > > >
> > > > In other words, shmem_mcopy_atomic_pte() duplicates a case it already
> > > > handles. So, expose it, and let shmem_mcopy_atomic_pte() use it
> > > > directly, to reduce code duplication.
> > > >
> > > > This requires that we refactor shmem_mcopy_atomic_pte() a bit:
> > > >
> > > > Instead of doing accounting (shmem_recalc_inode() et al) part-way
> > > > through the PTE setup, do it afterward. This frees up
> > > > mcopy_atomic_install_pte() from having to care about this accounting,
> > > > and means we don't need to e.g. shmem_uncharge() in the error path.
> > > >
> > > > A side effect is this switches shmem_mcopy_atomic_pte() to use
> > > > lru_cache_add_inactive_or_unevictable() instead of just lru_cache_add().
> > > > This wrapper does some extra accounting in an exceptional case, if
> > > > appropriate, so it's actually the more correct thing to use.
> > > >
> > > > Signed-off-by: Axel Rasmussen <[email protected]>
> > >
> > > Not quite. Two things.
> > >
> > > One, in this version, delete_from_page_cache(page) has vanished
> > > from the particular error path which needs it.
> >
> > Agreed. I also spotted that the set_page_dirty() seems to have been overlooked
> > when reusing mcopy_atomic_install_pte(), which afaiu should be move into the
> > helper.
>
> I think this is covered: we explicitly call SetPageDirty() just before
> returning in shmem_mcopy_atomic_pte(). If I remember correctly from a
> couple of revisions ago, we consciously put it here instead of in the
> helper because it resulted in simpler code (error handling in
> particular, I think?), and not all callers of the new helper need it.

Indeed, yes that looks okay.

>
> >
> > >
> > > Two, and I think this predates your changes (so needs a separate
> > > fix patch first, for backport to stable? a user with bad intentions
> > > might be able to trigger the BUG), in pondering the new error paths
> > > and that /* don't free the page */ one in particular, isn't it the
> > > case that the shmem_inode_acct_block() on entry might succeed the
> > > first time, but atomic copy fail so -ENOENT, then something else
> > > fill up the tmpfs before the retry comes in, so that retry then
> > > fail with -ENOMEM, and hit the BUG_ON(page) in __mcopy_atomic()?
> > >
> > > (As I understand it, the shmem_inode_unacct_blocks() has to be
> > > done before returning, because the caller may be unable to retry.)
> > >
> > > What the right fix is rather depends on other uses of __mcopy_atomic():
> > > if they obviously cannot hit that BUG_ON(page), you may prefer to leave
> > > it in, and fix it here where shmem_inode_acct_block() fails. Or you may
> > > prefer instead to delete that "else BUG_ON(page);" - looks as if that
> > > would end up doing the right thing. Peter may have a preference.
> >
> > To me, the BUG_ON(page) wanted to guarantee mfill_atomic_pte() should have
> > consumed the page properly when possible. Removing the BUG_ON() looks good
> > already, it will just stop covering the case when e.g. ret==0.
> >
> > So maybe slightly better to release the page when shmem_inode_acct_block()
> > fails (so as to still keep some guard on the page)?
>
> This second issue, I will take some more time to investigate. :)

No worry - take your time. :)

--
Peter Xu

2021-04-28 20:52:26

by Peter Xu

[permalink] [raw]
Subject: Re: [PATCH v5 09/10] userfaultfd/selftests: reinitialize test context in each test

On Tue, Apr 27, 2021 at 03:52:43PM -0700, Axel Rasmussen wrote:
> Currently, the context (fds, mmap-ed areas, etc.) are global. Each test
> mutates this state in some way, in some cases really "clobbering it"
> (e.g., the events test mremap-ing area_dst over the top of area_src, or
> the minor faults tests overwriting the count_verify values in the test
> areas). We run the tests in a particular order, each test is careful to
> make the right assumptions about its starting state, etc.
>
> But, this is fragile. It's better for a test's success or failure to not
> depend on what some other prior test case did to the global state.
>
> To that end, clear and reinitialize the test context at the start of
> each test case, so whatever prior test cases did doesn't affect future
> tests.
>
> This is particularly relevant to this series because the events test's
> mremap of area_dst screws up assumptions the minor fault test was
> relying on. This wasn't a problem for hugetlb, as we don't mremap in
> that case.
>
> Signed-off-by: Axel Rasmussen <[email protected]>

Reviewed-by: Peter Xu <[email protected]>

--
Peter Xu

2021-04-28 20:53:54

by Peter Xu

[permalink] [raw]
Subject: Re: [PATCH v5 10/10] userfaultfd/selftests: exercise minor fault handling shmem support

On Tue, Apr 27, 2021 at 03:52:44PM -0700, Axel Rasmussen wrote:
> Enable test_uffdio_minor for test_type == TEST_SHMEM, and modify the
> test slightly to pass in / check for the right feature flags.
>
> Signed-off-by: Axel Rasmussen <[email protected]>

Reviewed-by: Peter Xu <[email protected]>

--
Peter Xu