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 <[email protected]> Cc: Steve Sistare <[email protected]> Cc: Muchun Song <[email protected]> Cc: David Hildenbrand <[email protected]> Cc: Andrew Morton <[email protected]> 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
