Re: [PATCH RFC 1/1] mm/filemap: handle large folio split race in page cache lookups

[Kiryl Shutsemau]

On Thu, Mar 05, 2026 at 07:24:38PM +0000, Matthew Wilcox wrote:
> On Thu, Mar 05, 2026 at 12:34:33PM -0600, Chris J Arges wrote:
> > We have been hitting VM_BUG_ON_FOLIO(!folio_contains(folio, index)) in
> > production environments. These machines are using XFS with large folio
> > support enabled and are under high memory pressure.
> > 
> > >From reading the code it seems plausible that folio splits due to memory
> > reclaim are racing with filemap_fault() serving mmap page faults.
> > 
> > The existing code checks for truncation (folio->mapping != mapping) and
> > retries, but there does not appear to be equivalent handling for the
> > split case. The result is:
> > 
> >   kernel BUG at mm/filemap.c:3519!
> >   VM_BUG_ON_FOLIO(!folio_contains(folio, index), folio)
> 
> This didn't occur to me as a possibility because filemap_get_entry()
> is _supposed_ to take care of it.  But if this patch fixes it, then
> we need to understand why it works.
> 
> folio_split() needs to be sure that it's the only one holding a reference
> to the folio.  To that end, it calculates the expected refcount of the
> folio, and freezes it (sets the refcount to 0 if the refcount is the
> expected value).  Once filemap_get_entry() has incremented the refcount,
> freezing will fail.
> 
> But of course, we can race.  filemap_get_entry() can load a folio first,
> the entire folio_split can happen, then it calls folio_try_get() and
> succeeds, but it no longer covers the index we were looking for.  That's
> what the xas_reload() is trying to prevent -- if the index is for a
> folio which has changed, then the xas_reload() should come back with a
> different folio and we goto repeat.
> 
> So how did we get through this with a reference to the wrong folio?

What would xas_reload() return if we raced with split and index pointed
to a tail page before the split?

Wouldn't it return the folio that was a head and check will pass?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov