Re: [PATCH v2 2/2] mm/memory: refuse to map page cache folios that are not uptodate

[Kiryl Shutsemau]

On Wed, Nov 11, 2026 at 09:05:54AM +0100, David Hildenbrand (Arm) wrote:
> On 11/9/26 13:04, Kiryl Shutsemau (Meta) wrote:
> > A page cache folio that is not uptodate holds data that was never read
> > in or cleared. shmem fallocate(2), for one, adds such folios and clears
> > them only on first use. Mapping one hands userspace whatever the memory
> > held before.
> > 
> > Each path that maps page cache checks for this itself: fault-around in
> > next_uptodate_folio(), a fault in filemap_fault() or shmem_get_folio().
> > Nothing in the mapping code does, so a caller that misses the check
> > leaks stale memory silently. MADV_COLLAPSE did exactly that with
> > fallocated shmem THPs until recently.
> > 
> > Warn and refuse the mapping in __do_fault() and do_set_pmd().
> > 
> > __do_fault() checks the folio ->fault() returned while it is still
> > locked. do_shared_fault() unlocks it for ->page_mkwrite(), and a
> > writeback error can clear PG_uptodate after that; that case retries the
> > fault instead. For CoW, __do_fault() sees the page cache folio that
> > do_cow_fault() copies from. do_set_pmd() covers MADV_COLLAPSE, which
> > maps without a fault.
> > 
> > The fault gets SIGBUS, as for a folio that fails to read in.
> > MADV_COLLAPSE fails and leaves the range to be faulted in.
> > 
> > Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> > Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> > ---
> 
> Reviewed-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> 
> How do we proceed with patch #1 (on which this patch relies on :) )?

I will move the helper from 1/2 here and send v3 as a single patch.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov