Re: [PATCH v2 1/5] mm: limit pagecache folio refcount when mapping pages

[Zi Yan]

On Sat Nov 7, 2026 at 5:23 PM EST, 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; on 4k
> kernels it would be more processes than usually allowed on a system, and
> on 16k or 64k kernels the processes alone would consume hundreds of GiB.
> Something to be sorted out at some point, but it's not as urgent.
>
> Being able to overflow the refcount is 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.
>
> During fork(), prefer not copying page table entries over failing fork.
> Export __folio_ref_count_over_limit() right away as that will be needed
> from fuse code next. Use has_capability_noaudit(current, CAP_SYS_RESOURCE),
> so filesystem credential overrides do not affect access to the admin
> reserve, and to not generate auditing messages simply because we access
> optional reserves.
>
> 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.
>
> Check order 0 folios as well 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.
>
> Note: When zapping file migration entries, uffd-wp is preserved using PTE
> markers, but uffd-rwp protection is lost because there is no corresponding
> marker. try_to_unmap() already has the same problem when unmapping file
> folios, so we'll happily ignore it.
>
> Note2: For now we don't let migration or PTE table collapse exceed any
> limits. Admins can always refault, and if we are already operating in
> admin reserves, something is already off.
>
> Reported-by: Chris Mason <mason@xxxxxxxxxx>
> 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@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---
>  include/linux/mm.h | 30 ++++++++++++++++++++++++++++++
>  mm/filemap.c       |  6 ++++++
>  mm/huge_memory.c   | 29 +++++++++++++++++++++++++++--
>  mm/memory.c        | 35 +++++++++++++++++++++++++++++++----
>  mm/migrate.c       | 11 +++++++++++
>  mm/userfaultfd.c   |  4 ++++
>  mm/util.c          | 12 ++++++++++++
>  7 files changed, 121 insertions(+), 6 deletions(-)
>

LGTM. I wonder if we want to add a VM_WARN_ON_ONCE() refcount check in
__folio_add_file_rmap() and __folio_dup_file_rmap(), so that no new
caller will not miss the refcount limit handling. That also requires a
reproducer for verification, but it might be hard to trigger all
possible code path. Anyway,

Acked-by: Zi Yan <ziy@xxxxxxxxxx>

-- 
Best Regards,
Yan, Zi