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

Barry Song
2024-03-13T07:20:01Z

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!

> ---
> 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.

> +
> + 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.

> }
>
> /*
> --
> 2.25.1
>

Overall, LGTM,

Reviewed-by: Barry Song <v-songbaohua@oppo.com>