[PATCH v4 0/3] mm/memfd: Reserve hugetlb folios before allocation

[Vivek Kasireddy]

There are cases when we try to pin a folio but discover that it has
not been faulted-in. So, we try to allocate it in memfd_alloc_folio()
but the allocation request may not succeed if there are no active
reservations in the system at that instant.

Therefore, making a reservation (by calling hugetlb_reserve_pages())
associated with the allocation will ensure that our request would
not fail due to lack of reservations. This will also ensure that
proper region/subpool accounting is done with our allocation.

-----------------------------

Patchset overview:

Patch 1: Return nr of updated entries from hugetlb_reserve_pages()
Patch 2: Make reservation (hugetlb_reserve_pages) in memfd_alloc_folio()
Patch 3: New udmabuf selftest to invoke memfd_alloc_folio()

This series is tested by running the new udmabuf selftest introduced
in patch #3 along with the other selftests.

Changelog:
v3 -> v4:
- Create a standalone patch to fix the BUG reported by syzbot that
  can be backported to stable kernels (Andrew)
- Split the changes in memfd_alloc_folio() that add a call to
  hugetlb_reserve_pages() into a separate patch

v2 -> v3:
- Call hugetlb_unreserve_pages() only if the reservation was actively
  (and successfully) made from memfd_alloc_folio() (David)

v1 -> v2:
- Replace VM_BUG_ON() with WARN_ON_ONCE() in the function
  alloc_hugetlb_folio_reserve() (David)
- Move the inline function subpool_inode() from hugetlb.c into the
  relevant header (hugetlb.h)
- Call hugetlb_unreserve_pages() if the folio cannot be added to
  the page cache as well
- Added a new udmabuf selftest to exercise the same path as that
  of syzbot

Cc: Gerd Hoffmann <kraxel at redhat.com>
Cc: Steve Sistare <steven.sistare at oracle.com>
Cc: Muchun Song <muchun.song at linux.dev>
Cc: David Hildenbrand <david at redhat.com>
Cc: Andrew Morton <akpm at linux-foundation.org>

Vivek Kasireddy (3):
  mm/hugetlb: Make hugetlb_reserve_pages() return nr of entries updated
  mm/memfd: Reserve hugetlb folios before allocation
  selftests/udmabuf: Add a test to pin first before writing to memfd

 fs/hugetlbfs/inode.c                          |  8 +++----
 include/linux/hugetlb.h                       |  7 +++++-
 mm/hugetlb.c                                  | 24 ++++++++++---------
 mm/memfd.c                                    | 17 ++++++++++---
 .../selftests/drivers/dma-buf/udmabuf.c       | 20 +++++++++++++++-
 5 files changed, 56 insertions(+), 20 deletions(-)

-- 
2.49.0