Re: [PATCH v3] mm/hugetlb: fix a deadlock with pagecache_folio and hugetlb_fault_mutex_table

[James Houghton <jthoughton@google.com>]

On Wed, May 28, 2025 at 11:45 AM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Wed, May 28, 2025 at 05:09:26PM +0200, David Hildenbrand wrote:
> > On 28.05.25 17:03, Peter Xu wrote:
> > > So I'm not 100% sure we need the folio lock even for copy; IIUC a refcount
> > > would be enough?
> >
> > The introducing patches seem to talk about blocking concurrent migration /
> > rmap walks.
>
> I thought the main reason was because PageLock protects us against writes,
> so when copying (in case of copying the underlying file), we want the
> file to be stable throughout the copy?
>
> > Maybe also concurrent fallocate(PUNCH_HOLE) is a problem regarding
> > reservations? Not sure ...
>
> fallocate()->hugetlb_vmdelete_list() tries to grab the vma in write-mode,
> and hugetlb_wp() grabs the lock in read-mode, so we should be covered?
>
> Also, hugetlbfs_punch_hole()->remove_inode_hugepages() will try to grab the mutex.
>
> The only fishy thing I see is hugetlbfs_zero_partial_page().
>
> But that is for old_page, and as I said, I thought main reason was to
> protect us against writes during the copy.
>
> > For 2) I am also not sure if we need need the pagecache folio locked; I
> > doubt it ... but this code is not the easiest to follow.
>
> I have been staring at that code and thinking about potential scenarios
> for a few days now, and I cannot convice myself that we need
> pagecache_folio's lock when pagecache_folio != old_folio because as a
> matter of fact I cannot think of anything it protects us against.

Hi Oscar,

Have you thought about the UFFDIO_CONTINUE case (hugetlb_mfill_atomic_pte())?

I'm slightly concerned that, if you aren't holding pagecache_folio's
lock, there might be issues where hugetlb_mfill_atomic_pte() proceeds
to map a hugetlb page that it is not supposed to. (For example, if the
fault handler does not generally hold pagecache_folio's lock,
hugetlb_mfill_atomic_pte() will see a page in the pagecache and map
it, even though it may not have been zeroed yet.)

I haven't had enough time to fully think through this case, but just
want to make sure it has been considered.

Thanks!

> I plan to rework this in a more sane way, or at least less offusctaed, and then
> Galvin can fire his syzkaller to check whether we are good.
>
> --
> Oscar Salvador
> SUSE Labs
>