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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 27.05.25 10:06, Dev Jain wrote: > > On 27/05/25 1:18 pm, David Hildenbrand wrote: >> On 27.05.25 05:20, Dev Jain wrote: >>> >>> On 26/05/25 11:58 pm, Shivank Garg wrote: >>>> hpage_collapse_scan_file() calls is_refcount_suitable(), which in turn >>>> calls folio_mapcount(). folio_mapcount() checks folio_test_large() >>>> before >>>> proceeding to folio_large_mapcount(), but there is a race window >>>> where the >>>> folio may get split/freed between these checks, triggering: >>>> >>>>     VM_WARN_ON_FOLIO(!folio_test_large(folio), folio) >>>> >>>> Take a temporary reference to the folio in hpage_collapse_scan_file(). >>>> This stabilizes the folio during refcount check and prevents incorrect >>>> large folio detection due to concurrent split/free. Use helper >>>> folio_expected_ref_count() + 1 to compare with folio_ref_count() >>>> instead of using is_refcount_suitable(). >>>> >>>> Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single >>>> value") >>>> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com >>>> Closes: >>>> https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com >>>> >>>> Suggested-by: David Hildenbrand >>>> Acked-by: David Hildenbrand >>>> Signed-off-by: Shivank Garg >>>> --- >>> >>> The patch looks fine. >>> >>> I was just wondering about the implications of this on migration. >>> Earlier >>> we had a refcount race between migration and shmem page fault via >>> filemap_get_entry() >>> taking a reference and not releasing it till we take the folio lock, >>> which was held >>> by the migration path. I would like to *think* that real workloads, >>> when migrating >>> pages, will *not* be faulting on those pages simultaneously, just >>> guessing. But now >>> we have a kernel thread (khugepaged) racing against migration. I may >>> just be over-speculating. >> >> I'm not quite sure I understand the concern you have. Any temporary >> reference can temporarily block migration, however, the retry logic >> should be able to handle that just fine -- and this code is not really >> special (see filemap_get_entry()). > > > You are correct that any temp ref can block migration, however, that > reference has to come after the folios have been isolated in the > migration path. > > So the probability of someone taking a reference on the folio is quite > low since it has been isolated. The problem with filemap_get_entry() is > that it finds > > the folio in the pagecache, so isolation is useless, then takes a > reference and then shmem_get_folio_gfp() does a folio_lock() instead of > folio_try_lock(). > > This was the race which I talked about an year back at [1]. My concern > is that we are adding another candidate to that race; just wondering if > there is > > a better solution to fix the race mentioned in Shivank's patchset. Note that in this code here, we're not locking the folio just yet, we're only grabbing a reference for a very short time. We will isolate+lock the folios in collapse_file(), where we also lock all slots in the pagecache. The alternative would be to also lock all slots here, which is arguably worse. -- Cheers, David / dhildenb