Re: [PATCH 1/5] mm: limit pagecache folio refcount when mapping pages

[Lance Yang <lance.yang@linux.dev>]

On Thu, Nov 05, 2026 at 02:10:10PM +0100, David Hildenbrand (Arm) wrote:
>With large file folios, overflowing the folio mapcount and refcount
>became possible. For anon folios it is currently not really an issue:
>each large anon folio can only be mapped once into a process.
>
>That's not really big news, but it's about time that we do something
>about it because some modern technology might find ways to abuse it :)
>
>One of the enablers for that was commit ddc58f27f9ee ("mm: drop tail page
>refcounting"), however, file thp support was added later.
>
>File folios can be mapped multiple times into the same process, allowing
>for processes to hold many references to the same folio.
>
>As a short-term solution, let's introduce two folio refcount limits:
>one for ordinary consumers and one for admins. Leave some headroom
>before the refcount goes negative, which we really don't want (having
>the mapcount go negative is also not expected).
>
>Start reporting SIGBUS if we run into limits. When restoring mappings
>from migration entries, instead zap the pte/pmd entry. We allow for
>slightly exceeding the limit to keep things simpler.
>
>This is far from ideal, but better than all easy alternatives we can
>backport comfortably. For now it fixes the big problem: letting
>the refcount (+mapcount) go negative and cause trouble.
>
>This is only a temporary fix, we'll follow up with something better.
>
>While the refcount checks are not required for order 0 folios, let's
>just add them for consistency: each folio_add_file_rmap*() and
>folio_dup_file_rmap*() needs a prior check.
>
>Also, just document -EHWPOISON for copy_present_ptes() while at it.
>
>Reported-by: Chris Mason <mason@kernel.org>
>Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
>Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
>Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
>Cc: stable@vger.kernel.org
>Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>---
> include/linux/mm.h | 33 +++++++++++++++++++++++++++++++++
> mm/filemap.c       |  6 ++++++
> mm/huge_memory.c   | 24 ++++++++++++++++++++++--
> mm/memory.c        | 23 ++++++++++++++++++++---
> mm/migrate.c       | 11 +++++++++++
> mm/userfaultfd.c   |  4 ++++
> 6 files changed, 96 insertions(+), 5 deletions(-)

Nice work! I just went through all folio_add_file_rmap*() and
folio_dup_file_rmap*() callers, including callers of set_pte_range().

All covered, looks good to me1

Reviewed-by: Lance Yang <lance.yang@linux.dev>