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

[Matthew Wilcox <willy@infradead.org>]

On Thu, Dec 25, 2025 at 05:04:38PM +1300, 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!

Heh, thanks.  My mailserver is actually in a different timezone from me,
so it was still the 24th when I sent it ;-)

> 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.

Fair enough.  I hadn't noticed the SHORT_SYMLINK_LEN case when I
wrote this.  That does change things somewhat, but still for a 160 byte
symlink, we have 5 cachelines of data and 59 cachelines of zeroes.

> 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.

It already happens, it's just camouflaged.  owner_2 is the same bit as
mappedtodisk.