[PATCH 0/5] mm: limit pagecache folio refcount

[David Hildenbrand (Arm)]

Users can create a lot of folio references to pagecache folios, making
it possible to overflow the refcount of large folios.

While we knew that one could in theory overflow a folio's refcount, we
mostly assumed that bad things would happen and it wouldn't go
unnoticed.

Not so sure if that is really always the case, and leaving that problem
unsolved is irresponsible, so let's limit pagecache folio refcounts.

This is really only relevant for large folios / thps in the page cache.
With anon folios (including THPs), we should not be
able to trigger folio refcount overflows (until we get PUD-sized ones ;) ).

It was likely a problem for anon folios as well before we had a
dedicated pincount.

Let's limit the refcount before it turns negative, giving admin processes
some extra reserves (to reover a problematic system). We still leave some
headroom to allow for some slight limit overshooting, to make the
implementation easier.

Then, we also check when splicing pagecache folios to a pipe, and
improve our existing checks in gup. We also have to take care of
mmu_gather, to not hold too many references when zapping (whereby new
page faults could remap pages concurrently).

This is only a temporary solution for upstream+stable, until we have
something better. These fixes will be a bit challenging to backport to
stable, but it will be much easier compared to the RFC series I have here
were we do things properly by (what we discussed a couple of times
earlier):

 - Letting the mapcount of large folios be a long
 - Holding a single refcount if the folio is mapped, across all mappings
 - Reverting the limits introduced in this series

I'll polish that up and send it once we merged something that can be
backported to stable.

Tested on x86-64 by lowering the folio refcount limits such that limits
can easily be exceeded, to then test fault (SIGBUS), fork(), splice and GUp
behavior briefly when we run into refcount limits on a shmem THP.

Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
---
David Hildenbrand (Arm) (5):
      mm: limit pagecache folio refcount when mapping pages
      mm: limit pagecache folio refcount when splicing into pipes
      mm/mmu_gather: flush pending references early before the folio limit
      iommufd: fix cleanup when adding memfd folio pins fails
      mm/gup: limit pagecache folio refcount when grabbing references

 drivers/iommu/iommufd/pages.c | 15 +++++++++++----
 include/linux/mm.h            | 43 ++++++++++++++++++++++++++++++++++++++++---
 mm/filemap.c                  | 10 ++++++++++
 mm/gup.c                      | 12 ++++++++----
 mm/huge_memory.c              | 24 ++++++++++++++++++++++--
 mm/memory.c                   | 27 ++++++++++++++++++++++-----
 mm/migrate.c                  | 11 +++++++++++
 mm/mmu_gather.c               |  5 +++++
 mm/shmem.c                    |  4 ++++
 mm/userfaultfd.c              |  4 ++++
 10 files changed, 137 insertions(+), 18 deletions(-)

---

base-commit: 02f72aa35c5e7dff443c62cd9964686396c8b201

change-id: 20261105-refcount_overflow_stable-542e024130fe

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Cheers,

David