Re: [PATCH 2/5] mm/khugepaged: scan mm's more efficiently under mmap_lock contention

[Lance Yang <lance.yang@linux.dev>]

On Tue, Nov 03, 2026 at 08:27:43AM +0000, Lorenzo Stoakes (ARM) wrote:
>Ostensibly khugepaged collapse traverses eligible mm's, scanning up to
>/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan eligible pages
>at a time, sleeping for /sys/.../scan_sleep_millisecs between each
>traversal.
>
>mm's are scanned in collapse_scan_mm_slot() under the mmap read lock,
>obtained using mmap_read_trylock(), skipping the mm if there is lock
>contention, and dropping the lock before allocation.
>
>khugepaged will keep scanning the same mm until it is about to be torn
>down, prctl(PR_SET_THP_DISABLE) disables THP for it, mm->mmap_lock
>contention occurs, or all its VMAs have been scanned.
>
>When any of these occur, khugepaged doesn't keep track of where it left
>off, as the current address is tracked in
>khugepaged_scan.address (essentially a global value), which is reset.
>
>This is a problem when lock contention occurs, as when the mm is next
>scanned, it starts from address 0 again.
>
>If there's a lot of mmap_lock contention this can significantly slow down
>collapse later in the mm, since the /sys/.../pages_to_scan budget can run
>out before the end of the mm is reached.
>
>The budget is incremented (as tracked in cc->progress) at a rate of 1 page
>per ineligible VMA, but the budget is only checked once eligible VMAs are
>encountered.
>
>Then, it only takes the combined total of ineligible VMAs and eligible but
>non-collapsible pages to exceed the /sys/.../pages_to_scan budget for the
>rest of the mm to be skipped.
>
>For a system with 4 KiB page size and default sysctl settings this only
>requires 4096 ineligible VMAs/uncollapsible pages to be skipped before this
>happens.
>
>Examples of eligible but uncollapsible ranges are those containing a large
>number of forked PTE entries (more than /sys/.../max_ptes_shared), swapped
>out PTE entries (more than /sys/.../max_ptes_swap) or none or zero-page PTE
>entries (more than /sys/.../max_ptes_none).
>
>The latter might seem unlikely to happen given /sys/.../max_ptes_none
>defaults to HPAGE_PMD_NR - 1 and CONFIG_PT_RECLAIM frees PTE page tables,
>however CONFIG_PT_RECLAIM only removes the page table if the zapped range
>spans at least a PMD size (see pte_table_reclaim_possible()).
>
>Thus, userland software that applies madvise(..., MADV_DONTNEED) across
>sub-PMD ranges can thwart this, which is quite feasible for memory
>allocators and GC heaps, for instance.
>
>Resolve the issue by simply tracking the last examined address in the
>khugepaged_mm_slot object, so when the mm is resumed, the scan starts from
>where it left off.
>
>Additionally, skip to the next candidate if a VMA cannot be collapsed.
>
>This issue was noticed during some work in which an mm was instantiated
>with a large number of small VMAs under significant mmap_lock contention,
>during which collapse was observed to take 10.8s. With this patch applied
>it took 3.6s to collapse the same mm.
>
>Also, while here, update a couple VM_BUG_ON()'s to VM_WARN_ON_ONCE()'s.
>
>This patch is also a pre-requisite for a later patch in this series.
>
>Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>---

Looks good on its own, since vmi isn't reset inside the loop yet. Patch
#05 will need to handle the ALIGN() overflow Sashiko reported before
adding that reset.

Reviewed-by: Lance Yang <lance.yang@linux.dev>