Re: [PATCH 0/4] mm/ksm: use per-VMA locking for find_mergeable_vma()

[Jinjiang Tu]

在 2026/9/11 16:04, xu.xin16@xxxxxxxxxx 写道:
> From: Xu Xin (ZTE) <xu.xin@xxxxxxxxx>
>
> KSM scans VM_MERGEABLE VMAs and currently protects each scan with
> mmap_read_lock(). Per-VMA locking allows KSM to read-lock only the VMA
> it is actually interested in, so that unrelated mmap()/munmap() activity
> in the same mm no longer blocks ksmd.
>
> This series is organized as follows:
>
>    Patch 1 removes an unused 'vma' member from struct folio_walk.  It has
>    never been used since its introduction and is pure cleanup.
>
>    Patch 2 adds a 'walk_lock' member to struct folio_walk and extends
>    folio_walk_start() to assert the required locking mode.  Existing
>    callers are converted to pass PGWALK_RDLOCK, so there is no functional
>    change.  This prepares folio_walk_start() for callers that hold a
>    per-VMA read lock instead of mmap_read_lock(), which is needed by the
>    Patch 4. No functional change.
>
>    Patch 3 tranforms the boolean 'lock_vma' into the enum 'page_walk_lock'
>    without any behavior changed, which is prepared for the Patch 4 to use
>    per-VMA locking. No functional change.
>
>    Patch 4 introduces find_mergeable_vma_locked(), which uses the
>    universal per-VMA locking helper vma_start_read_unlocked() to look up
>    and read-lock a VM_MERGEABLE VMA without taking mmap_read_lock().  All
>    KSM call sites that previously used find_mergeable_vma() under
>    mmap_read_lock() are converted to the new helper, and the locking in
>    get_mergeable_page() is switched to PGWALK_VMA_RDLOCK_VERIFY so that
>    folio_walk_start() can verify the per-VMA lock is held.
>
> A microbenchmark was run to measure the time KSM takes to merge a
> victim region under mmap_lock contention. Under interference from 4 churner
> threads, the merge time of the per-VMA KSM-optimized kernel is
> significantly reduced by 50%.

Hi.

During task exiting, __ksm_exit() uses mmap_write_lock() to synchronize with ksmd.
see the comment of ksm_test_exit().

void __ksm_exit(struct mm_struct *mm)
{
  ...

	if (easy_to_free) {
		mm_slot_free(mm_slot_cache, mm_slot);
		mm_flags_clear(MMF_VM_MERGE_ANY, mm);
		mm_flags_clear(MMF_VM_MERGEABLE, mm);
		mmdrop(mm);
	} else if (mm_slot) {
		mmap_write_lock(mm);
		mmap_write_unlock(mm);
	}

}

When ksmd currently is scanning the exiting mm, we should guarantee the mm pagetable
still valid (i.e., mm_users > 0). However, ksm_mm_slot only holds mm_count, which only
guarantees the mm_strcut isn't freed. So, __ksm_exit() uses mmap write lock to synchronize
with ksmd.

IIUC, vma_read_lock cannot be exclusive with mmap_write_lock().

> Xu Xin (4):
>    mm/pagewalk: delete the unused member
>    mm: make folio_walk_start()'s locking asserts scalable
>    mm/ksm: make break_ksm() more scalable
>    mm/ksm: add find_mergeable_vma_locked() to use per-VMA locking
>
>   arch/s390/mm/fault.c     |  4 +-
>   include/linux/pagewalk.h |  2 +-
>   kernel/events/uprobes.c  |  4 +-
>   mm/huge_memory.c         |  4 +-
>   mm/ksm.c                 | 85 ++++++++++++++++++++++++----------------
>   mm/migrate.c             |  8 +++-
>   mm/pagewalk.c            | 10 ++++-
>   mm/rmap.c                |  4 +-
>   8 files changed, 79 insertions(+), 42 deletions(-)