Re: [PATCH v2 2/2] mm: don't allocate a page table under RCU in do_set_pmd()

[Kiryl Shutsemau <kas@kernel.org>]

On Fri, Nov 06, 2026 at 01:36:13PM +0100, David Hildenbrand (Arm) wrote:
> do_fault_around() skips PTE table preallocation when the PMD already points
> to a PTE table. A concurrent collapse can then retract that table and
> clear the PMD before filemap_map_pages() locks the new PMD-sized folio.
> 
> filemap_map_pmd() can then call do_set_pmd() with an empty PMD and no
> preallocated table. On PowerPC hash MMU, do_set_pmd() then tries
> allocating a PTE table to deposit. But filemap_map_pmd() calls
> do_set_pmd() under RCU, so allocating from that context is not good.
> 
> This race became possible when retract_page_tables() stopped taking the
> mmap and VMA locks. Previously, those locks excluded retraction while
> the fault was in progress.
> 
> Add a may_alloc argument to do_set_pmd() and pass false from
> filemap_map_pmd(), so we can fail with VM_FAULT_FALLBACK instead of
> trying to allocate.
> 
> This issue should be fairly hard to hit in practice.
> 
> Fixes: 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock")
> Cc: stable@vger.kernel.org
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov