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.

[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

Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: stable@vger.kernel.org
Fixes: 7e2a5e5ab217 ("mm: migrate: use __folio_test_movable()")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
---
 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);
 
 	if (folio_test_writeback(src)) {
 		/*
--

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

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@googlegroups.com>
> Cc: stable@vger.kernel.org
> Fixes: 7e2a5e5ab217 ("mm: migrate: use __folio_test_movable()")
> Signed-off-by: Jeongjun Park <aha310510@gmail.com>
> ---
>   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@redhat.com>


-- 
Cheers,

David / dhildenb

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.

> 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@googlegroups.com>
> > Cc: stable@vger.kernel.org
> > Fixes: 7e2a5e5ab217 ("mm: migrate: use __folio_test_movable()")
> > Signed-off-by: Jeongjun Park <aha310510@gmail.com>
> > ---
> >   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@redhat.com>
> 
> 
> -- 
> Cheers,
> 
> David / dhildenb
>

> Matthew Wilcox <willy@infradead.org> 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@googlegroups.com>
>>> Cc: stable@vger.kernel.org
>>> Fixes: 7e2a5e5ab217 ("mm: migrate: use __folio_test_movable()")
>>> Signed-off-by: Jeongjun Park <aha310510@gmail.com>
>>> ---
>>>  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@redhat.com>
>> 
>> 
>> --
>> Cheers,
>> 
>> David / dhildenb
>> 

On Tue, Sep 24, 2024 at 09:28:44AM +0900, Jeongjun Park wrote:
> > Matthew Wilcox <willy@infradead.org> 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 .

Because it can't be changed in a way which changes the value of
__folio_test_movable().  We have a refcount on the folio at this point,
so it can't be freed.  And __folio_set_movable() happens at allocation.

> Matthew Wilcox <willy@infradead.org> wrote:
> 
> On Tue, Sep 24, 2024 at 09:28:44AM +0900, Jeongjun Park wrote:
>>> Matthew Wilcox <willy@infradead.org> 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 .
> 
> Because it can't be changed in a way which changes the value of
> __folio_test_movable().  We have a refcount on the folio at this point,
> so it can't be freed.  And __folio_set_movable() happens at allocation.
> 

Thanks for the explanation. Then it seems appropriate to annotate 
data-race in __folio_test_movable() so that KCSAN ignores it.

I will apply the change and send you a new patch.

Regards,
Jeongjun Park