[PATCH] mm: fix race between MADV_FREE reclaim and blkdev direct IO read

Mauricio Faria de Oliveira
2021-12-11T02:21:28Z

Problem:
=======

Userspace might read the zero-page instead of actual data from a
direct IO read on a block device if the buffers have been called
madvise(MADV_FREE) on earlier (this is discussed below) due to a
race between page reclaim on MADV_FREE and blkdev direct IO read.

Race condition:
==============

During page reclaim, the MADV_FREE page check in try_to_unmap_one()
checks if the page is not dirty, then discards its PTE (vs remap it
back if the page is dirty).

However, after try_to_unmap_one() returns to shrink_page_list(), it
might keep the page _anyway_ if page_ref_freeze() fails (it expects
a single page ref from the isolation).

Well, blkdev_direct_IO() gets references for all pages, and on READ
operations it sets them dirty later.

So, if MADV_FREE pages (i.e., not dirty) are used as buffers (more
later) for direct IO read from block devices and page reclaim runs
during __blkdev_direct_IO[_simple]() AFTER bio_iov_iter_get_pages()
but BEFORE it sets pages dirty, that situation happens.

The direct IO read eventually completes. Now, when userspace reads
the buffers, the PTE is no longer there and the page fault handler
do_anonymous_page() services that with the zero-page, NOT the data!

A synthetic reproducer is provided.

Page faults:
===========

The data read from the block device probably won't generate faults
due to DMA (no MMU) but even in the case it wouldn't use DMA, that
happens on different virtual addresses (not user-mapped addresses)
because `struct bio_vec` stores `struct page` to figure addresses
out (which are different from/unrelated to user-mapped addresses)
for the data read.

Thus userspace reads (to user-mapped addresses) still fault, then
do_anonymous_page() gets another `struct page` that would address/
map to other memory than the `struct page` used by `struct bio_vec`
for the read (which runs correctly as the page wasn't freed due to
page_ref_freeze(), and is reclaimed later) -- but even if the page
addresses matched, that handler maps the zero-page in the PTE, not
that page's memory (on read faults.)

If page reclaim happens BEFORE bio_iov_iter_get_pages() the issue
doesn't happen, because that faults-in all pages as writeable, so
do_anonymous_page() sets up a new page/rmap/PTE, and that is used
by direct IO. The userspace reads don't fault as the PTE is there
(thus zero-page is not used.)

Solution:
========

One solution is to check for the expected page reference count in
try_to_unmap_one() too, which should be exactly two: one from the
isolation (checked by shrink_page_list()), and the other from the
rmap (dropped by the discard: label). If that doesn't match, then
remap the PTE back, just like page dirty does.

The new check in try_to_unmap_one() should be safe in races with
bio_iov_iter_get_pages() in get_user_pages() fast and slow paths,
as it's done under the PTE lock. The fast path doesn't take that
lock but it checks the PTE has changed, then drops the reference
and leaves the page for the slow path (which does take that lock).

- try_to_unmap_one()
- page_vma_mapped_walk()
- map_pte() # see pte_offset_map_lock():
pte_offset_map()
spin_lock()
- page_ref_count() # new check
- page_vma_mapped_walk_done() # see pte_unmap_unlock():
pte_unmap()
spin_unlock()

- bio_iov_iter_get_pages()
- __bio_iov_iter_get_pages()
- iov_iter_get_pages()
- get_user_pages_fast()
- internal_get_user_pages_fast()

# fast path
- lockless_pages_from_mm()
- gup_{pgd,p4d,pud,pmd,pte}_range()
ptep = pte_offset_map() # not _lock()
pte = ptep_get_lockless(ptep)
page = pte_page(pte)
try_grab_compound_head(page) # get ref
if (pte_val(pte) != pte_val(*ptep))
put_compound_head(page) # put ref
# leave page for slow path
# slow path
- __gup_longterm_unlocked()
- get_user_pages_unlocked()
- __get_user_pages_locked()
- __get_user_pages()
- follow_{page,p4d,pud,pmd}_mask()
- follow_page_pte()
ptep = pte_offset_map_lock()
pte = *ptep
page = vm_normal_page(pte)
try_grab_page(page) # get ref
pte_unmap_unlock()

Regarding transparent hugepages, that number shouldn't change, as
MADV_FREE (aka lazyfree) pages are PageAnon() && !PageSwapBacked()
(madvise_free_pte_range() -> mark_page_lazyfree() -> lru_lazyfree_fn())
thus should reach shrink_page_list() -> split_huge_page_to_list()
before try_to_unmap[_one](), so it deals with normal pages only.

(And in case unlikely/TTU_SPLIT_HUGE_PMD/split_huge_pmd_address()
happens, which it should not or be rare, the page refcount is not
two, as the head page counts tail pages, and tail pages have zero.
That also prevents checking the head `page` then incorrectly call
page_remove_rmap(subpage) for a tail page, that isn't even in the
shrink_page_list()'s page_list (an effect of split huge pmd/pmvw),
as it might happen today in this unlikely scenario.)

MADV_FREE'd buffers:
===================

So, back to the "if MADV_FREE pages are used as buffers" note.
The case is arguable, and subject to multiple interpretations.

The madvise(2) manual page on the MADV_FREE advice value says:
- 'After a successful MADV_FREE ... data will be lost when
the kernel frees the pages.'
- 'the free operation will be canceled if the caller writes
into the page' / 'subsequent writes ... will succeed and
then [the] kernel cannot free those dirtied pages'
- 'If there is no subsequent write, the kernel can free the
pages at any time.'

Thoughts, questions, considerations...
- Since the kernel didn't actually free the page (page_ref_freeze()
failed), should the data not have been lost? (on userspace read.)
- Should writes performed by the direct IO read be able to cancel
the free operation?
- Should the direct IO read be considered as 'the caller' too,
as it's been requested by 'the caller'?
- Should the bio technique to dirty pages on return to userspace
(bio_check_pages_dirty() is called/used by __blkdev_direct_IO())
be considered in another/special way here?
- Should an upcoming write from a previously requested direct IO
read be considered as a subsequent write, so the kernel should
not free the pages? (as it's known at the time of page reclaim.)

Technically, the last point would seem a reasonable consideration
and balance, as the madvise(2) manual page apparently (and fairly)
seem to assume that 'writes' are memory access from the userspace
process (not explicitly considering writes from the kernel or its
corner cases; again, fairly).. plus the kernel fix implementation
for the corner case of the largely 'non-atomic write' encompassed
by a direct IO read operation, is relatively simple; and it helps.

Reproducer:
==========

@ test.c (simplified, but works)

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/mman.h>

int main() {
int fd, i;
char *buf;

fd = open(DEV, O_RDONLY | O_DIRECT);

buf = mmap(NULL, BUF_SIZE, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

for (i = 0; i < BUF_SIZE; i += PAGE_SIZE)
buf[i] = 1; // init to non-zero

madvise(buf, BUF_SIZE, MADV_FREE);

read(fd, buf, BUF_SIZE);

for (i = 0; i < BUF_SIZE; i += PAGE_SIZE)
printf("%p: 0x%x\n", &buf[i], buf[i]);

return 0;
}

@ block/fops.c (formerly fs/block_dev.c)

+#include <linux/swap.h>
...
... __blkdev_direct_IO[_simple](...)
{
...
+ if (!strcmp(current->comm, "good"))
+ shrink_all_memory(ULONG_MAX);
+
ret = bio_iov_iter_get_pages(...);
+
+ if (!strcmp(current->comm, "bad"))
+ shrink_all_memory(ULONG_MAX);
...
}

@ shell

# yes | dd of=test.img bs=1k count=16
# DEV=$(losetup -f --show test.img)
# gcc -DDEV=\"$DEV\" -DBUF_SIZE=16384 -DPAGE_SIZE=4096 test.c -o test

# od -tx1 $DEV
0000000 79 0a 79 0a 79 0a 79 0a 79 0a 79 0a 79 0a 79 0a
*
0040000

# mv test good
# ./good
0x7f1509206000: 0x79
0x7f1509207000: 0x79
0x7f1509208000: 0x79
0x7f1509209000: 0x79

# mv good bad
# ./bad
0x7fd87272f000: 0x0
0x7fd872730000: 0x0
0x7fd872731000: 0x0
0x7fd872732000: 0x0

Ceph/TCMalloc:
=============

For documentation purposes, the use case driving the analysis/fix
is Ceph on Ubuntu 18.04, as the TCMalloc library there still uses
MADV_FREE to release unused memory to the system from the mmap'ed
page heap (might be committed back/used again; it's not munmap'ed.)
- PageHeap::DecommitSpan() -> TCMalloc_SystemRelease() -> madvise()
- PageHeap::CommitSpan() -> TCMalloc_SystemCommit() -> do nothing.

Note: TCMalloc switched back to MADV_DONTNEED a few commits after
the release in Ubuntu 18.04 (google-perftools/gperftools 2.5), so
the issue just 'disappeared' on Ceph on later Ubuntu releases but
is still present in the kernel, and can be hit by other use cases.

The observed issue seems to be the old Ceph bug #22464 [1], where
checksum mismatches are observed (and instrumentation with buffer
dumps shows zero-pages read from mmap'ed/MADV_FREE'd page ranges).

The issue in Ceph was reasonably deemed a kernel bug (comment #50)
and mostly worked around with a retry mechanism, but other parts
of Ceph could still hit that (rocksdb). Anyway, it's less likely
to be hit again as TCMalloc switched out of MADV_FREE by default.

(Some kernel versions/reports from the Ceph bug, and relation with
the MADV_FREE introduction/changes; TCMalloc versions not checked.)
- 4.4 good
- 4.5 (madv_free: introduction)
- 4.9 bad
- 4.10 good? maybe a swapless system
- 4.12 (madv_free: no longer free instantly on swapless systems)
- 4.13 bad

[1] https://tracker.ceph.com/issues/22464

Thanks:
======

Several people contributed to analysis/discussions/tests/reproducers
in the first stages when drilling down on ceph/tcmalloc/linux kernel:

- Dan Hill <daniel.hill@canonical.com>
- Dan Streetman <dan.streetman@canonical.com>
- Dongdong Tao <dongdong.tao@canonical.com>
- Gavin Guo <gavin.guo@canonical.com>
- Gerald Yang <gerald.yang@canonical.com>
- Heitor Alves de Siqueira <halves@canonical.com>
- Ioanna Alifieraki <ioanna-maria.alifieraki@canonical.com>
- Jay Vosburgh <jay.vosburgh@canonical.com>
- Matthew Ruffell <matthew.ruffell@canonical.com>
- Ponnuvel Palaniyappan <ponnuvel.palaniyappan@canonical.com>

Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
---

P.S.: sorry for the very long commit message; hopefully it might
provide enough context and considerations on the problem and fix
approach to help reviewers. Tested on v5.16-rc4.

mm/rmap.c | 13 ++++++++++++-
mm/vmscan.c | 2 +-
2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 163ac4e6bcee..f04151aae03b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1570,7 +1570,18 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,

/* MADV_FREE page check */
if (!PageSwapBacked(page)) {
- if (!PageDirty(page)) {
+ int refcount = page_ref_count(page);
+
+ /*
+ * The only page refs must be from the isolation
+ * (checked by the caller shrink_page_list() too)
+ * and the (single) rmap (dropped by discard:).
+ *
+ * Check the reference count before dirty flag
+ * with memory barrier; see __remove_mapping().
+ */
+ smp_rmb();
+ if (refcount == 2 && !PageDirty(page)) {
/* Invalidate as we cleared the pte */
mmu_notifier_invalidate_range(mm,
address, address + PAGE_SIZE);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index fb9584641ac7..c1ea4e14f510 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1688,7 +1688,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
mapping = page_mapping(page);
}
} else if (unlikely(PageTransHuge(page))) {
- /* Split file THP */
+ /* Split file/lazyfree THP */
if (split_huge_page_to_list(page, page_list))
goto keep_locked;
}
--
2.32.0