Re: [PATCH] mm/shmem: fix uninitialized folio in shmem_symlink

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 1/5/26 9:58 PM, Brian Foster wrote:
> On Thu, Dec 25, 2025 at 06:08:08PM +0800, Baolin Wang wrote:
>>
>>
>> On 2025/12/25 12:04, Barry Song wrote:
>>> On Thu, Dec 25, 2025 at 6:01 AM Matthew Wilcox <willy@infradead.org> wrote:
>>>>
>>>> On Wed, Dec 24, 2025 at 10:40:27PM +1300, Barry Song wrote:
>>>>> From: Barry Song <baohua@kernel.org>
>>>>>
>>>>> Uninitialized folio allocated in shmem_symlink() may be accessed
>>>>> during swap-out, causing KMSAN BUG:
>>>>
>>>> This would be an unfortunate way to fix it.  The vast majority of
>>>> symlinks are short, and we'll never access past the \0 in normal
>>>> operation, so we'll be dirtying a lot of cachelines essentially to (1)
>>>> shut up an automated tool and (2) optimise a corner case.
>>>>
>>>> How about this instead which delays zeroing to swapout?
>>>
>>> Matthew, thank you very much for your review, even during Christmas.
>>> I would like to wish you a happy holiday!
>>>
>>> I am not quite sure, as shm symlinks do not seem very common. Since
>>> allocating a folio requires a symname longer than 128 bytes (where
>>> 128 == SHORT_SYMLINK_LEN), such cases appear even rarer.
>>>
>>> BTW, do we need to migrate the owner_2 flag in folio_migrate_flags()?
>>> If so, I am not quite sure it is worth changing the hotpath to
>>> accommodate this.
>>
>> +1. At least for me, using the 'PG_owner_2' flag alone to mark this uncommon
>> case doesn't seem quite worthwhile.
>>
> 
> Also JFYI the post-eof swapout zeroing work (still pending) looks to me
> like it would cover the swapout time case [1]. That's just if you wanted
> to go that route here; creation time zeroing for the large symlink case
> seems reasonable enough to me as well.

IMHO, your post-eof zeroing work is intended to zero !uptodate or beyond 
EOF folios before swap-out, however, the symlink folio is uptodate and 
not beyond the EOF. I am not sure if it will make your code more 
complicated when you want to cover this symlink case.

Why I prefer Barry's fix: First, the symlink folio is marked Uptodate 
after copying the symlink name, but the whole folio hasn’t been 
initialized, which seems unreasonable to me. Second, as I said before, 
using the 'PG_owner_2' flag to mark this uncommon case doesn’t seem 
worthwhile. Currently, IIUC the 'PG_owner_2' is only used by btrfs; if 
we ever want to remove the 'PG_owner_2', this uncommon symlink case 
shouldn’t block its removal.

BTW, as I said before, I've reviewed and tested your post-eof work[1]. 
Maybe you could resend the patch set so that Hugh can take a look when 
he has time.

> [1] https://lore.kernel.org/linux-mm/20251121152246.1023918-3-bfoster@redhat.com/
> 
>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>> index ec6c01378e9d..f3b3be1b50fe 100644
>>>> --- a/mm/shmem.c
>>>> +++ b/mm/shmem.c
>>>> @@ -1636,6 +1636,13 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>>>>                   folio_mark_uptodate(folio);
>>>>           }
>>>>
>>>> +       /* Zero out symlink tails to help with compression */
>>>> +       if (folio_test_owner_2(folio)) {
>>>> +               struct inode *inode = folio->mapping->host;
>>>> +               folio_zero_segment(folio, inode->i_size, folio_size(folio));
>>>> +               folio_clear_owner_2(folio);
>>>> +       }
>>>> +
>>>>           if (!folio_alloc_swap(folio)) {
>>>>                   bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages);
>>>>                   int error;
>>>> @@ -4133,6 +4140,7 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
>>>>                   memcpy(folio_address(folio), symname, len);
>>>>                   folio_mark_uptodate(folio);
>>>>                   folio_mark_dirty(folio);
>>>> +               folio_set_owner_2(folio);
>>>>                   folio_unlock(folio);
>>>>                   folio_put(folio);
>>>>           }
>>>
>>> Thanks
>>> Barry
>>
>>