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

[David Hildenbrand <david@redhat.com>]

On 16.04.25 10:51, David Hildenbrand wrote:
> On 16.04.25 10:41, Baolin Wang wrote:
>>
>>
>> On 2025/4/16 16:22, David Hildenbrand wrote:
>>> On 16.04.25 08:32, Baolin Wang wrote:
>>>>
>>>>
>>>> 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.
>>>
>>> This patch does not change the existing behavior. But I stumbled over
>>> that as well ... and scratched my head.
>>>
>>>>
>>>>     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?
>>>
>>> I think what the doc really meant is "PMD-mapped THP". PTE-mapped THPs
>>> always had the same behavior: per PTE of the THP we would increment
>>> nr_failed by 1.
>>
>> No? For pte-mapped THPs, it only adds 1 for the large folio, since we
>> have below check in queue_folios_pte_range().
>>
>> if (folio == qp->large)
>> 	continue;
>>
>> Or I missed anything else?
> 
> Ah, I got confused by that and thought it would only be for LRU
> isolation purposes.
> 
> Yeah, it will kind-of work for now and I think you are correct that we
> would only increment nr_failed by 1.
> 
> I still think that counting nr_failed that way is dubious. We should be
> counting pages, which is something that user space from migrate_pages()
> could understand. Having it count arbitrary THPs/large folio sizes is
> really questionable.
> 
> But that is indeed a separate issue to resolve.

Digging into it:

commit 1cb5d11a370f661c5d0d888bb0cfc2cdc5791382
Author: Hugh Dickins <hughd@google.com>
Date:   Tue Oct 3 02:17:43 2023 -0700

     mempolicy: fix migrate_pages(2) syscall return nr_failed
     
     "man 2 migrate_pages" says "On success migrate_pages() returns the number
     of pages that could not be moved".  Although 5.3 and 5.4 commits fixed
     mbind(MPOL_MF_STRICT|MPOL_MF_MOVE*) to fail with EIO when not all pages
     could be moved (because some could not be isolated for migration),
     migrate_pages(2) was left still reporting only those pages failing at the
     migration stage, forgetting those failing at the earlier isolation stage.
     
     Fix that by accumulating a long nr_failed count in struct queue_pages,
     returned by queue_pages_range() when it's not returning an error, for
     adding on to the nr_failed count from migrate_pages() in mm/migrate.c.  A
     count of pages?  It's more a count of folios, but changing it to pages
     would entail more work (also in mm/migrate.c): does not seem justified.

Yeah, we should be counting pages, but likely nobody really cares, because we
only care if everything was migrated or something was not migrated.

-- 
Cheers,

David / dhildenb