Re: [PATCH v4 6/6] mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD

[Ryan Roberts <ryan.roberts@arm.com>]

On 13/03/2024 09:16, Barry Song wrote:
> On Wed, Mar 13, 2024 at 10:03 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 13/03/2024 07:19, Barry Song wrote:
>>> On Tue, Mar 12, 2024 at 4:01 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>
>>>> Rework madvise_cold_or_pageout_pte_range() to avoid splitting any large
>>>> folio that is fully and contiguously mapped in the pageout/cold vm
>>>> range. This change means that large folios will be maintained all the
>>>> way to swap storage. This both improves performance during swap-out, by
>>>> eliding the cost of splitting the folio, and sets us up nicely for
>>>> maintaining the large folio when it is swapped back in (to be covered in
>>>> a separate series).
>>>>
>>>> Folios that are not fully mapped in the target range are still split,
>>>> but note that behavior is changed so that if the split fails for any
>>>> reason (folio locked, shared, etc) we now leave it as is and move to the
>>>> next pte in the range and continue work on the proceeding folios.
>>>> Previously any failure of this sort would cause the entire operation to
>>>> give up and no folios mapped at higher addresses were paged out or made
>>>> cold. Given large folios are becoming more common, this old behavior
>>>> would have likely lead to wasted opportunities.
>>>>
>>>> While we are at it, change the code that clears young from the ptes to
>>>> use ptep_test_and_clear_young(), which is more efficent than
>>>> get_and_clear/modify/set, especially for contpte mappings on arm64,
>>>> where the old approach would require unfolding/refolding and the new
>>>> approach can be done in place.
>>>>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>>
>>> This looks so much better than our initial RFC.
>>> Thank you for your excellent work!
>>
>> Thanks - its a team effort - I had your PoC and David's previous batching work
>> to use as a template.
>>
>>>
>>>> ---
>>>>  mm/madvise.c | 89 ++++++++++++++++++++++++++++++----------------------
>>>>  1 file changed, 51 insertions(+), 38 deletions(-)
>>>>
>>>> diff --git a/mm/madvise.c b/mm/madvise.c
>>>> index 547dcd1f7a39..56c7ba7bd558 100644
>>>> --- a/mm/madvise.c
>>>> +++ b/mm/madvise.c
>>>> @@ -336,6 +336,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>>>>         LIST_HEAD(folio_list);
>>>>         bool pageout_anon_only_filter;
>>>>         unsigned int batch_count = 0;
>>>> +       int nr;
>>>>
>>>>         if (fatal_signal_pending(current))
>>>>                 return -EINTR;
>>>> @@ -423,7 +424,8 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>>>>                 return 0;
>>>>         flush_tlb_batched_pending(mm);
>>>>         arch_enter_lazy_mmu_mode();
>>>> -       for (; addr < end; pte++, addr += PAGE_SIZE) {
>>>> +       for (; addr < end; pte += nr, addr += nr * PAGE_SIZE) {
>>>> +               nr = 1;
>>>>                 ptent = ptep_get(pte);
>>>>
>>>>                 if (++batch_count == SWAP_CLUSTER_MAX) {
>>>> @@ -447,55 +449,66 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>>>>                         continue;
>>>>
>>>>                 /*
>>>> -                * Creating a THP page is expensive so split it only if we
>>>> -                * are sure it's worth. Split it if we are only owner.
>>>> +                * If we encounter a large folio, only split it if it is not
>>>> +                * fully mapped within the range we are operating on. Otherwise
>>>> +                * leave it as is so that it can be swapped out whole. If we
>>>> +                * fail to split a folio, leave it in place and advance to the
>>>> +                * next pte in the range.
>>>>                  */
>>>>                 if (folio_test_large(folio)) {
>>>> -                       int err;
>>>> -
>>>> -                       if (folio_estimated_sharers(folio) > 1)
>>>> -                               break;
>>>> -                       if (pageout_anon_only_filter && !folio_test_anon(folio))
>>>> -                               break;
>>>> -                       if (!folio_trylock(folio))
>>>> -                               break;
>>>> -                       folio_get(folio);
>>>> -                       arch_leave_lazy_mmu_mode();
>>>> -                       pte_unmap_unlock(start_pte, ptl);
>>>> -                       start_pte = NULL;
>>>> -                       err = split_folio(folio);
>>>> -                       folio_unlock(folio);
>>>> -                       folio_put(folio);
>>>> -                       if (err)
>>>> -                               break;
>>>> -                       start_pte = pte =
>>>> -                               pte_offset_map_lock(mm, pmd, addr, &ptl);
>>>> -                       if (!start_pte)
>>>> -                               break;
>>>> -                       arch_enter_lazy_mmu_mode();
>>>> -                       pte--;
>>>> -                       addr -= PAGE_SIZE;
>>>> -                       continue;
>>>> +                       const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
>>>> +                                               FPB_IGNORE_SOFT_DIRTY;
>>>> +                       int max_nr = (end - addr) / PAGE_SIZE;
>>>> +
>>>> +                       nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
>>>> +                                            fpb_flags, NULL);
>>>
>>> I wonder if we have a quick way to avoid folio_pte_batch() if users
>>> are doing madvise() on a portion of a large folio.
>>
>> Good idea. Something like this?:
>>
>>         if (pte_pfn(pte) == folio_pfn(folio)
> 
> what about
> 
> "If (pte_pfn(pte) == folio_pfn(folio) && max_nr >= nr_pages)"
> 
>  just to account for cases where the user's end address falls within
> the middle of a large folio?

yes, even better. I'll add this for the next version.

> 
> 
> BTW, another minor issue is here:
> 
>                 if (++batch_count == SWAP_CLUSTER_MAX) {
>                         batch_count = 0;
>                         if (need_resched()) {
>                                 arch_leave_lazy_mmu_mode();
>                                 pte_unmap_unlock(start_pte, ptl);
>                                 cond_resched();
>                                 goto restart;
>                         }
>                 }
> 
> We are increasing 1 for nr ptes, thus, we are holding PTL longer
> than small folios case? we used to increase 1 for each PTE.
> Does it matter?

I thought about that, but the vast majority of the work is per-folio, not
per-pte. So I concluded it would be best to continue to increment per-folio.

> 
>>                 nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
>>                                      fpb_flags, NULL);
>>
>> If we are not mapping the first page of the folio, then it can't be a full
>> mapping, so no need to call folio_pte_batch(). Just split it.
>>
>>>
>>>> +
>>>> +                       if (nr < folio_nr_pages(folio)) {
>>>> +                               int err;
>>>> +
>>>> +                               if (folio_estimated_sharers(folio) > 1)
>>>> +                                       continue;
>>>> +                               if (pageout_anon_only_filter && !folio_test_anon(folio))
>>>> +                                       continue;
>>>> +                               if (!folio_trylock(folio))
>>>> +                                       continue;
>>>> +                               folio_get(folio);
>>>> +                               arch_leave_lazy_mmu_mode();
>>>> +                               pte_unmap_unlock(start_pte, ptl);
>>>> +                               start_pte = NULL;
>>>> +                               err = split_folio(folio);
>>>> +                               folio_unlock(folio);
>>>> +                               folio_put(folio);
>>>> +                               if (err)
>>>> +                                       continue;
>>>> +                               start_pte = pte =
>>>> +                                       pte_offset_map_lock(mm, pmd, addr, &ptl);
>>>> +                               if (!start_pte)
>>>> +                                       break;
>>>> +                               arch_enter_lazy_mmu_mode();
>>>> +                               nr = 0;
>>>> +                               continue;
>>>> +                       }
>>>>                 }
>>>>
>>>>                 /*
>>>>                  * Do not interfere with other mappings of this folio and
>>>> -                * non-LRU folio.
>>>> +                * non-LRU folio. If we have a large folio at this point, we
>>>> +                * know it is fully mapped so if its mapcount is the same as its
>>>> +                * number of pages, it must be exclusive.
>>>>                  */
>>>> -               if (!folio_test_lru(folio) || folio_mapcount(folio) != 1)
>>>> +               if (!folio_test_lru(folio) ||
>>>> +                   folio_mapcount(folio) != folio_nr_pages(folio))
>>>>                         continue;
>>>
>>> This looks so perfect and is exactly what I wanted to achieve.
>>>
>>>>
>>>>                 if (pageout_anon_only_filter && !folio_test_anon(folio))
>>>>                         continue;
>>>>
>>>> -               VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
>>>> -
>>>> -               if (!pageout && pte_young(ptent)) {
>>>> -                       ptent = ptep_get_and_clear_full(mm, addr, pte,
>>>> -                                                       tlb->fullmm);
>>>> -                       ptent = pte_mkold(ptent);
>>>> -                       set_pte_at(mm, addr, pte, ptent);
>>>> -                       tlb_remove_tlb_entry(tlb, pte, addr);
>>>> +               if (!pageout) {
>>>> +                       for (; nr != 0; nr--, pte++, addr += PAGE_SIZE) {
>>>> +                               if (ptep_test_and_clear_young(vma, addr, pte))
>>>> +                                       tlb_remove_tlb_entry(tlb, pte, addr);
>>>> +                       }
>>>
>>> This looks so smart. if it is not pageout, we have increased pte
>>> and addr here; so nr is 0 and we don't need to increase again in
>>> for (; addr < end; pte += nr, addr += nr * PAGE_SIZE)
>>>
>>> otherwise, nr won't be 0. so we will increase addr and
>>> pte by nr.
>>
>> Indeed. I'm hoping that Lance is able to follow a similar pattern for
>> madvise_free_pte_range().
>>
>>
>>>
>>>
>>>>                 }
>>>>
>>>>                 /*
>>>> --
>>>> 2.25.1
>>>>
>>>
>>> Overall, LGTM,
>>>
>>> Reviewed-by: Barry Song <v-songbaohua@oppo.com>
>>
>> Thanks!
>>
>>