> Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Sep 23, 2024 at 05:56:40PM +0200, David Hildenbrand wrote:
>>> On 22.09.24 17:17, Jeongjun Park wrote:
>>> I found a report from syzbot [1]
>>>
>>> When __folio_test_movable() is called in migrate_folio_unmap() to read
>>> folio->mapping, a data race occurs because the folio is read without
>>> protecting it with folio_lock.
>>>
>>> This can cause unintended behavior because folio->mapping is initialized
>>> to a NULL value. Therefore, I think it is appropriate to call
>>> __folio_test_movable() under the protection of folio_lock to prevent
>>> data-race.
>>
>> We hold a folio reference, would we really see PAGE_MAPPING_MOVABLE flip?
>> Hmm
>
> No; this shows a page cache folio getting truncated. It's fine; really
> a false alarm from the tool. I don't think the proposed patch
> introduces any problems, but it's all a bit meh.
>
Well, I still don't understand why it's okay to read folio->mapping
without folio_lock . Since migrate_folio_unmap() is already protected
by folio_lock , I think it's definitely necessary to fix it to read
folio->mapping under folio_lock protection. If it were still okay to
call __folio_test_movable() without folio_lock , then we could
annotate data-race, but I'm still not sure if this is a good way
to do it.
Regards,
Jeongjun Park
>> Even a racing __ClearPageMovable() would still leave PAGE_MAPPING_MOVABLE
>> set.
>>
>>> [1]
>>>
>>> ==================================================================
>>> BUG: KCSAN: data-race in __filemap_remove_folio / migrate_pages_batch
>>>
>>> write to 0xffffea0004b81dd8 of 8 bytes by task 6348 on cpu 0:
>>> page_cache_delete mm/filemap.c:153 [inline]
>>> __filemap_remove_folio+0x1ac/0x2c0 mm/filemap.c:233
>>> filemap_remove_folio+0x6b/0x1f0 mm/filemap.c:265
>>> truncate_inode_folio+0x42/0x50 mm/truncate.c:178
>>> shmem_undo_range+0x25b/0xa70 mm/shmem.c:1028
>>> shmem_truncate_range mm/shmem.c:1144 [inline]
>>> shmem_evict_inode+0x14d/0x530 mm/shmem.c:1272
>>> evict+0x2f0/0x580 fs/inode.c:731
>>> iput_final fs/inode.c:1883 [inline]
>>> iput+0x42a/0x5b0 fs/inode.c:1909
>>> dentry_unlink_inode+0x24f/0x260 fs/dcache.c:412
>>> __dentry_kill+0x18b/0x4c0 fs/dcache.c:615
>>> dput+0x5c/0xd0 fs/dcache.c:857
>>> __fput+0x3fb/0x6d0 fs/file_table.c:439
>>> ____fput+0x1c/0x30 fs/file_table.c:459
>>> task_work_run+0x13a/0x1a0 kernel/task_work.c:228
>>> resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
>>> exit_to_user_mode_loop kernel/entry/common.c:114 [inline]
>>> exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline]
>>> __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
>>> syscall_exit_to_user_mode+0xbe/0x130 kernel/entry/common.c:218
>>> do_syscall_64+0xd6/0x1c0 arch/x86/entry/common.c:89
>>> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>
>>> read to 0xffffea0004b81dd8 of 8 bytes by task 6342 on cpu 1:
>>> __folio_test_movable include/linux/page-flags.h:699 [inline]
>>> migrate_folio_unmap mm/migrate.c:1199 [inline]
>>> migrate_pages_batch+0x24c/0x1940 mm/migrate.c:1797
>>> migrate_pages_sync mm/migrate.c:1963 [inline]
>>> migrate_pages+0xff1/0x1820 mm/migrate.c:2072
>>> do_mbind mm/mempolicy.c:1390 [inline]
>>> kernel_mbind mm/mempolicy.c:1533 [inline]
>>> __do_sys_mbind mm/mempolicy.c:1607 [inline]
>>> __se_sys_mbind+0xf76/0x1160 mm/mempolicy.c:1603
>>> __x64_sys_mbind+0x78/0x90 mm/mempolicy.c:1603
>>> x64_sys_call+0x2b4d/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:238
>>> do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>>> do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>>> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>
>>> value changed: 0xffff888127601078 -> 0x0000000000000000
>>
>> Note that this doesn't flip PAGE_MAPPING_MOVABLE, just some unrelated bits.
>>
>>>
>>> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> Fixes: 7e2a5e5ab217 ("mm: migrate: use __folio_test_movable()")
>>> Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
>>> ---
>>> mm/migrate.c | 3 ++-
>>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 923ea80ba744..e62dac12406b 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1118,7 +1118,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>>> int rc = -EAGAIN;
>>> int old_page_state = 0;
>>> struct anon_vma *anon_vma = NULL;
>>> - bool is_lru = !__folio_test_movable(src);
>>> + bool is_lru;
>>> bool locked = false;
>>> bool dst_locked = false;
>>> @@ -1172,6 +1172,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>>> locked = true;
>>> if (folio_test_mlocked(src))
>>> old_page_state |= PAGE_WAS_MLOCKED;
>>> + is_lru = !__folio_test_movable(src);
>>
>>
>> Looks straight forward, though
>>
>> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>>
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>
