Re: [PATCH v2 3/3] mm/khugepaged: release contended mmap read lock after scanning enough VMAs

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 11/6/26 4:57 AM, Lorenzo Stoakes (ARM) wrote:
> khugepaged walks an mm's VMAs under the mmap read lock in
> collapse_scan_mm_slot(), checking for ineligible VMAs first, and skipping
> them if encountered.
> 
> It charges cc->progress 1 page against the /sys/.../pages_to_scan budget
> when these are encountered, however only checking this budget once an
> eligible PMD-spanning VMA is encountered.
> 
> It can therefore hold the mmap read lock for a long time if there are a
> large number of ineligible VMAs present in an mm, over which time no mmap
> write locks (and thus no VMA write locks) can be acquired.
> 
> Address this by checking whether the mmap lock is contended after every
> KHUGEPAGED_MAX_VMA_SCAN_BATCH VMAs walked, that is a quarter of the
> pages_to_scan budget, and if so dropping and re-acquiring the mmap read
> lock before resuming the walk from the per-mm cursor.
> 
> As the walk is resumed from the cursor rather than the VMA that was current
> when the lock was dropped, the cursor must never move backwards. A VMA
> ending at the top of the address space causes ALIGN() to wrap the cursor to
> zero, so treat such a VMA as the last in the mm rather than skipping past
> it.
> 
> This was tested locally with ~1M ineligible VMAs mapped ahead of a
> collapsible region of memory, and resulted in a reduced mmap_lock hold time
> of 170us down from 95ms.
> 
> This was also observed to reduce mmap_lock stalls in concurrent threads of
> a similar duration. Note that a reader re-acquiring the lock can currently
> steal it from a writer that is queued but not yet running, so the full
> benefit to writers depends on the separate rwsem change at [0].
> 
> Local measurements indicated no change for ordinary use cases, so this is
> all upside.
> 
> This work came out of investigations into mmap stalls when getting latency
> numbers for an unrelated patch series.
> 
> [0]: https://lore.kernel.org/all/20261105-rwsem-dont-steal-from-queued-writer-v1-1-275e070f426f@kernel.org/
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---

[snip]

>   		if (!collapse_possible_orders(vma, vma->vm_flags, TVA_KHUGEPAGED)) {
> -			/* Unable to collapse, skip to the next candidate. */
> -			mm_slot->address = ALIGN(vma->vm_end, HPAGE_PMD_SIZE);
> +			const unsigned long next =
> +				ALIGN(vma->vm_end, HPAGE_PMD_SIZE);
> +
>   			cc->progress++;
> +			/* Check for overflow. */
> +			if (unlikely(next < vma->vm_end)) {
> +				vma = NULL;
> +				break;
> +			}
> +			/* Unable to collapse, skip to the next candidate. */
> +			mm_slot->address = next;
>   			continue;
>   		}

IMO, these changes seem to belong in patch 2?