The patch titled
Subject: mm/madvise: make MADV_POPULATE_(READ|WRITE) handle VM_FAULT_RETRY properly
has been added to the -mm mm-unstable branch. Its filename is
mm-madvise-make-madv_populate_readwrite-handle-vm_fault_retry-properly.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-madvise-make-madv_populate_readwrite-handle-vm_fault_retry-properly.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/madvise: make MADV_POPULATE_(READ|WRITE) handle VM_FAULT_RETRY properly
Date: Thu, 14 Mar 2024 17:12:59 +0100
Patch series "mm/madvise: make MADV_POPULATE_(READ|WRITE) handle
VM_FAULT_RETRY properly".
Derrick reports that in some cases where pread() would fail with -EIO and
mmap()+access would generate a SIGBUS signal, MADV_POPULATE_READ /
MADV_POPULATE_WRITE will keep retrying forever and not fail with -EFAULT.
It all boils down to missing VM_FAULT_RETRY handling. Let's try to handle
that in a better way, similar to how ordinary GUP handles it.
Details in patch #1. In short, move special MADV_POPULATE_(READ|WRITE)
VMA handling into __get_user_pages(), and make faultin_page_range() call
__get_user_pages_locked(), which handles VM_FAULT_RETRY. Further, avoid
the now-useless madvise VMA walk, because __get_user_pages() will perform
the VMA lookup either way.
I briefly played with handling the FOLL_MADV_POPULATE checks in
__get_user_pages() a bit differently, integrating them with existing
handling, but it ended up looking worse. So I decided to keep it simple.
Likely, we need better selftests, but the reproducer from Darrick might be
a bit hard to convert into a simple selftest.
Note that using mlock() in Darricks reproducer results in a similar
endless retry. Likely, that is not what we want, and we should handle
VM_FAULT_RETRY in populate_vma_page_range() / __mm_populate() as well.
However, similarly using __get_user_pages_locked() might be more
complicated, because of the advanced VMA handling in
populate_vma_page_range().
Further, most populate_vma_page_range() callers simply ignore the return
values, so it's unclear in which cases we expect to just silently fail, or
where we'd want to retry+fail or endlessly retry instead.
This patch (of 2):
Darrick reports that in some cases where pread() would fail with -EIO and
mmap()+access would generate a SIGBUS signal, MADV_POPULATE_READ /
MADV_POPULATE_WRITE will keep retrying forever and not fail with -EFAULT.
While the madvise() call can be interrupted by a signal, this is not the
desired behavior. MADV_POPULATE_READ / MADV_POPULATE_WRITE should behave
like page faults in that case: fail and not retry forever.
A reproducer can be found at [1].
The reason is that __get_user_pages(), as called by
faultin_vma_page_range(), will not handle VM_FAULT_RETRY in a proper way:
it will simply return 0 when VM_FAULT_RETRY happened, making
madvise_populate()->faultin_vma_page_range() retry again and again, never
setting FOLL_TRIED->FAULT_FLAG_TRIED for __get_user_pages().
__get_user_pages_locked() does what we want, but duplicating that logic in
faultin_vma_page_range() feels wrong.
So let's use __get_user_pages_locked() instead, that will detect
VM_FAULT_RETRY and set FOLL_TRIED when retrying, making the fault handler
return VM_FAULT_SIGBUS (VM_FAULT_ERROR) at some point, propagating -EFAULT
from faultin_page() to __get_user_pages(), all the way to
madvise_populate().
But, there is an issue: __get_user_pages_locked() will end up re-taking
the MM lock and then __get_user_pages() will do another VMA lookup. In
the meantime, the VMA layout could have changed and we'd fail with
different error codes than we'd want to.
As __get_user_pages() will currently do a new VMA lookup either way, let
it do the VMA handling in a different way, controlled by a new
FOLL_MADV_POPULATE flag, effectively moving these checks from
madvise_populate() + faultin_page_range() in there.
With this change, Darricks reproducer properly fails with -EFAULT, as
documented for MADV_POPULATE_READ / MADV_POPULATE_WRITE.
[1] https://lore.kernel.org/all/20240313171936.GN1927156@frogsfrogsfrogs/
Link: https://lkml.kernel.org/r/20240314161300.382526-1-david@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20240314161300.382526-2-david@xxxxxxxxxx
Fixes: 4ca9b3859dac ("mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault page tables")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Reported-by: Darrick J. Wong <djwong@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/20240311223815.GW1927156@frogsfrogsfrogs/
Cc: Darrick J. Wong <djwong@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/gup.c | 54 ++++++++++++++++++++++++++++--------------------
mm/internal.h | 10 +++++---
mm/madvise.c | 17 +--------------
3 files changed, 40 insertions(+), 41 deletions(-)
--- a/mm/gup.c~mm-madvise-make-madv_populate_readwrite-handle-vm_fault_retry-properly
+++ a/mm/gup.c
@@ -1206,6 +1206,22 @@ static long __get_user_pages(struct mm_s
/* first iteration or cross vma bound */
if (!vma || start >= vma->vm_end) {
+ /*
+ * MADV_POPULATE_(READ|WRITE) wants to handle VMA
+ * lookups+error reporting differently.
+ */
+ if (gup_flags & FOLL_MADV_POPULATE) {
+ vma = vma_lookup(mm, start);
+ if (!vma) {
+ ret = -ENOMEM;
+ goto out;
+ }
+ if (check_vma_flags(vma, gup_flags)) {
+ ret = -EINVAL;
+ goto out;
+ }
+ goto retry;
+ }
vma = gup_vma_lookup(mm, start);
if (!vma && in_gate_area(mm, start)) {
ret = get_gate_page(mm, start & PAGE_MASK,
@@ -1683,35 +1699,35 @@ long populate_vma_page_range(struct vm_a
}
/*
- * faultin_vma_page_range() - populate (prefault) page tables inside the
- * given VMA range readable/writable
+ * faultin_page_range() - populate (prefault) page tables inside the
+ * given range readable/writable
*
* This takes care of mlocking the pages, too, if VM_LOCKED is set.
*
- * @vma: target vma
+ * @mm: the mm to populate page tables in
* @start: start address
* @end: end address
* @write: whether to prefault readable or writable
* @locked: whether the mmap_lock is still held
*
- * Returns either number of processed pages in the vma, or a negative error
- * code on error (see __get_user_pages()).
+ * Returns either number of processed pages in the MM, or a negative error
+ * code on error (see __get_user_pages()). Note that this function reports
+ * errors related to VMAs, such as incompatible mappings, as expected by
+ * MADV_POPULATE_(READ|WRITE).
*
- * vma->vm_mm->mmap_lock must be held. The range must be page-aligned and
- * covered by the VMA. If it's released, *@locked will be set to 0.
+ * The range must be page-aligned.
+ *
+ * mm->mmap_lock must be held. If it's released, *@locked will be set to 0.
*/
-long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
- unsigned long end, bool write, int *locked)
+long faultin_page_range(struct mm_struct *mm, unsigned long start,
+ unsigned long end, bool write, int *locked)
{
- struct mm_struct *mm = vma->vm_mm;
unsigned long nr_pages = (end - start) / PAGE_SIZE;
int gup_flags;
long ret;
VM_BUG_ON(!PAGE_ALIGNED(start));
VM_BUG_ON(!PAGE_ALIGNED(end));
- VM_BUG_ON_VMA(start < vma->vm_start, vma);
- VM_BUG_ON_VMA(end > vma->vm_end, vma);
mmap_assert_locked(mm);
/*
@@ -1723,19 +1739,13 @@ long faultin_vma_page_range(struct vm_ar
* a poisoned page.
* !FOLL_FORCE: Require proper access permissions.
*/
- gup_flags = FOLL_TOUCH | FOLL_HWPOISON | FOLL_UNLOCKABLE;
+ gup_flags = FOLL_TOUCH | FOLL_HWPOISON | FOLL_UNLOCKABLE |
+ FOLL_MADV_POPULATE;
if (write)
gup_flags |= FOLL_WRITE;
- /*
- * We want to report -EINVAL instead of -EFAULT for any permission
- * problems or incompatible mappings.
- */
- if (check_vma_flags(vma, gup_flags))
- return -EINVAL;
-
- ret = __get_user_pages(mm, start, nr_pages, gup_flags,
- NULL, locked);
+ ret = __get_user_pages_locked(mm, start, nr_pages, NULL, locked,
+ gup_flags);
lru_add_drain();
return ret;
}
--- a/mm/internal.h~mm-madvise-make-madv_populate_readwrite-handle-vm_fault_retry-properly
+++ a/mm/internal.h
@@ -686,9 +686,8 @@ struct anon_vma *folio_anon_vma(struct f
void unmap_mapping_folio(struct folio *folio);
extern long populate_vma_page_range(struct vm_area_struct *vma,
unsigned long start, unsigned long end, int *locked);
-extern long faultin_vma_page_range(struct vm_area_struct *vma,
- unsigned long start, unsigned long end,
- bool write, int *locked);
+extern long faultin_page_range(struct mm_struct *mm, unsigned long start,
+ unsigned long end, bool write, int *locked);
extern bool mlock_future_ok(struct mm_struct *mm, unsigned long flags,
unsigned long bytes);
@@ -1127,10 +1126,13 @@ enum {
FOLL_FAST_ONLY = 1 << 20,
/* allow unlocking the mmap lock */
FOLL_UNLOCKABLE = 1 << 21,
+ /* VMA lookup+checks compatible with MADV_POPULATE_(READ|WRITE) */
+ FOLL_MADV_POPULATE = 1 << 22,
};
#define INTERNAL_GUP_FLAGS (FOLL_TOUCH | FOLL_TRIED | FOLL_REMOTE | FOLL_PIN | \
- FOLL_FAST_ONLY | FOLL_UNLOCKABLE)
+ FOLL_FAST_ONLY | FOLL_UNLOCKABLE | \
+ FOLL_MADV_POPULATE)
/*
* Indicates for which pages that are write-protected in the page table,
--- a/mm/madvise.c~mm-madvise-make-madv_populate_readwrite-handle-vm_fault_retry-properly
+++ a/mm/madvise.c
@@ -908,27 +908,14 @@ static long madvise_populate(struct vm_a
{
const bool write = behavior == MADV_POPULATE_WRITE;
struct mm_struct *mm = vma->vm_mm;
- unsigned long tmp_end;
int locked = 1;
long pages;
*prev = vma;
while (start < end) {
- /*
- * We might have temporarily dropped the lock. For example,
- * our VMA might have been split.
- */
- if (!vma || start >= vma->vm_end) {
- vma = vma_lookup(mm, start);
- if (!vma)
- return -ENOMEM;
- }
-
- tmp_end = min_t(unsigned long, end, vma->vm_end);
/* Populate (prefault) page tables readable/writable. */
- pages = faultin_vma_page_range(vma, start, tmp_end, write,
- &locked);
+ pages = faultin_page_range(mm, start, end, write, &locked);
if (!locked) {
mmap_read_lock(mm);
locked = 1;
@@ -949,7 +936,7 @@ static long madvise_populate(struct vm_a
pr_warn_once("%s: unhandled return value: %ld\n",
__func__, pages);
fallthrough;
- case -ENOMEM:
+ case -ENOMEM: /* No VMA or out of memory. */
return -ENOMEM;
}
}
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-madvise-make-madv_populate_readwrite-handle-vm_fault_retry-properly.patch
mm-madvise-dont-perform-madvise-vma-walk-for-madv_populate_readwrite.patch
