[PATCH v3 0/8] Per-VMA lock support for swap and userfaults

Suren Baghdasaryan
2023-06-27T04:23:28Z

When per-VMA locks were introduced in [1] several types of page faults
would still fall back to mmap_lock to keep the patchset simple. Among them
are swap and userfault pages. The main reason for skipping those cases was
the fact that mmap_lock could be dropped while handling these faults and
that required additional logic to be implemented.
Implement the mechanism to allow per-VMA locks to be dropped for these
cases.
First, change handle_mm_fault to drop per-VMA locks when returning
VM_FAULT_RETRY or VM_FAULT_COMPLETED to be consistent with the way
mmap_lock is handled. Then change folio_lock_or_retry (and rename it to
folio_lock_fault) to accept vm_fault, which will be used to indicate
mmap_lock/per-VMA lock's state upon exit. Finally allow swap and uffd
page faults to be handled under per-VMA locks by dropping per-VMA locks
when waiting for a folio, the same way it's done under mmap_lock.
Naturally, once VMA lock is dropped that VMA should be assumed unstable
and can't be used.

Changes since v2 posted at [2]
- Moved prerequisite patches to the beginning (first 2 patches)
- Added a new patch 3/8 to make per-VMA locks consistent with mmap_locks
by dropping it on VM_FAULT_RETRY or VM_FAULT_COMPLETED.
- Implemented folio_lock_fault in 4/8, per Matthew Wilcox
- Replaced VM_FAULT_VMA_UNLOCKED with FAULT_FLAG_LOCK_DROPPED vmf_flag in
5/8.
- Merged swap page fault handling patch with the one implementing wait for
a folio into 6/8, per Peter Xu

Note: patch 3/8 will cause a trivial merge conflict in arch/arm64/mm/fault.c
when applied over mm-unstable branch due to a patch from ARM64 tree [3]
which is missing in mm-unstable.

[1] https://lore.kernel.org/all/20230227173632.3292573-1-surenb@google.com/
[2] https://lore.kernel.org/all/20230609005158.2421285-1-surenb@google.com/
[3] https://lore.kernel.org/all/20230524131305.2808-1-jszhang@kernel.org/

Suren Baghdasaryan (8):
swap: remove remnants of polling from read_swap_cache_async
mm: add missing VM_FAULT_RESULT_TRACE name for VM_FAULT_COMPLETED
mm: drop per-VMA lock in handle_mm_fault if retrying or when finished
mm: replace folio_lock_or_retry with folio_lock_fault
mm: make folio_lock_fault indicate the state of mmap_lock upon return
mm: handle swap page faults under per-VMA lock
mm: drop VMA lock before waiting for migration
mm: handle userfaults under VMA lock

arch/arm64/mm/fault.c | 3 +-
arch/powerpc/mm/fault.c | 3 +-
arch/s390/mm/fault.c | 3 +-
arch/x86/mm/fault.c | 3 +-
fs/userfaultfd.c | 42 +++++++++++++------------
include/linux/mm_types.h | 4 ++-
include/linux/pagemap.h | 13 ++++----
mm/filemap.c | 55 +++++++++++++++++++--------------
mm/madvise.c | 4 +--
mm/memory.c | 66 +++++++++++++++++++++++++---------------
mm/swap.h | 1 -
mm/swap_state.c | 12 +++-----
12 files changed, 120 insertions(+), 89 deletions(-)

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2.41.0.178.g377b9f9a00-goog