Re: [PATCH v3] mempolicy: Optimize queue_folios_pte_range by PTE batching

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

On 2025/4/16 13:30, Dev Jain wrote:
> After the check for queue_folio_required(), the code only cares about the
> folio in the for loop, i.e the PTEs are redundant. Therefore, optimize
> this loop by skipping over a PTE batch mapping the same folio.
> 
> With a test program migrating pages of the calling process, which includes
> a mapped VMA of size 4GB with pte-mapped large folios of order-9, and
> migrating once back and forth node-0 and node-1, the average execution
> time reduces from 7.5 to 4 seconds, giving an approx 47% speedup.
> 
> v2->v3:
>   - Don't use assignment in if condition
> 
> v1->v2:
>   - Follow reverse xmas tree declarations
>   - Don't initialize nr
>   - Move folio_pte_batch() immediately after retrieving a normal folio
>   - increment nr_failed in one shot
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>   mm/mempolicy.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index b28a1e6ae096..4d2dc8b63965 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -566,6 +566,7 @@ static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk)
>   static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
>   			unsigned long end, struct mm_walk *walk)
>   {
> +	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>   	struct vm_area_struct *vma = walk->vma;
>   	struct folio *folio;
>   	struct queue_pages *qp = walk->private;
> @@ -573,6 +574,7 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
>   	pte_t *pte, *mapped_pte;
>   	pte_t ptent;
>   	spinlock_t *ptl;
> +	int max_nr, nr;
>   
>   	ptl = pmd_trans_huge_lock(pmd, vma);
>   	if (ptl) {
> @@ -586,7 +588,9 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
>   		walk->action = ACTION_AGAIN;
>   		return 0;
>   	}
> -	for (; addr != end; pte++, addr += PAGE_SIZE) {
> +	for (; addr != end; pte += nr, addr += nr * PAGE_SIZE) {
> +		max_nr = (end - addr) >> PAGE_SHIFT;
> +		nr = 1;
>   		ptent = ptep_get(pte);
>   		if (pte_none(ptent))
>   			continue;
> @@ -598,6 +602,10 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
>   		folio = vm_normal_folio(vma, addr, ptent);
>   		if (!folio || folio_is_zone_device(folio))
>   			continue;
> +		if (folio_test_large(folio) && max_nr != 1)
> +			nr = folio_pte_batch(folio, addr, pte, ptent,
> +					     max_nr, fpb_flags,
> +					     NULL, NULL, NULL);
>   		/*
>   		 * vm_normal_folio() filters out zero pages, but there might
>   		 * still be reserved folios to skip, perhaps in a VDSO.
> @@ -630,7 +638,7 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
>   		if (!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) ||
>   		    !vma_migratable(vma) ||
>   		    !migrate_folio_add(folio, qp->pagelist, flags)) {
> -			qp->nr_failed++;
> +			qp->nr_failed += nr;

Sorry for chiming in late, but I am not convinced that 'qp->nr_failed' 
should add 'nr' when isolation fails.

 From the comments of queue_pages_range():
"
* >0 - this number of misplaced folios could not be queued for moving
  *      (a hugetlbfs page or a transparent huge page being counted as 1).
"

That means if a large folio is failed to isolate, we should only add '1' 
for qp->nr_failed instead of the number of pages in this large folio. Right?