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

[Baolin Wang]

On 2025/12/25 12:04, Barry Song wrote:
> On Thu, Dec 25, 2025 at 6:01 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > On Wed, Dec 24, 2025 at 10:40:27PM +1300, Barry Song wrote:
> > > From: Barry Song <baohua@xxxxxxxxxx>
> > >
> > > 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.

> > 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