Re: [PATCH] mm: thp: don't have to lock page anymore when splitting PMD
From: Yang Shi
Date: Mon Mar 07 2022 - 19:03:36 EST
On Mon, Mar 7, 2022 at 3:43 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, 7 Mar 2022 09:24:58 +0100 David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > On 07.03.22 03:07, Andrew Morton wrote:
> > > On Fri, 4 Mar 2022 19:50:08 +0100 David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >
> > >> @Andrew, the last mail I received was
> > >>
> > >> + mm-huge_memory-remove-stale-locking-logic-from-__split_huge_pmd.patch
> > >> added to -mm tree
> > >>
> > >> The patch shows up in mmotm as
> > >>
> > >> #[merged]mm-huge_memory-remove-stale-locking-logic-from-__split_huge_pmd.patch
> > >>
> > >> ... which shouldn't be true.
> > >
> > > I guess I mislabelled the reason for dropping it. Should have been to-be-updated,
> > > due to https://lkml.kernel.org/r/CAHbLzkpbnQyHRckoRtbZoaLvANu92MY4kEsbKudaQ8MDUA3nVg@xxxxxxxxxxxxxx
> > >
> >
> > Let me clarify.
> >
> > 1. I sent [1] (9 patches)
> >
> > 2. You queued the 9 patches
> >
> > E.g., in "mmotm 2022-02-15-20-22 uploaded"
> >
> > * mm-optimize-do_wp_page-for-exclusive-pages-in-the-swapcache.patch
> > * mm-optimize-do_wp_page-for-fresh-pages-in-local-lru-pagevecs.patch
> > * mm-slightly-clarify-ksm-logic-in-do_swap_page.patch
> > * mm-streamline-cow-logic-in-do_swap_page.patch
> > * mm-huge_memory-streamline-cow-logic-in-do_huge_pmd_wp_page.patch
> > * mm-khugepaged-remove-reuse_swap_page-usage.patch
> > * mm-swapfile-remove-stale-reuse_swap_page.patch
> > * mm-huge_memory-remove-stale-page_trans_huge_mapcount.patch
> > * mm-huge_memory-remove-stale-locking-logic-from-__split_huge_pmd.patch
> >
> > 3. The last patch in the series was dropped. What remains are 8 patches.
> >
> > E.g., in "mmotm 2022-02-24-22-38 uploaded"
> >
> > * mm-optimize-do_wp_page-for-exclusive-pages-in-the-swapcache.patch
> > * mm-optimize-do_wp_page-for-fresh-pages-in-local-lru-pagevecs.patch
> > * mm-slightly-clarify-ksm-logic-in-do_swap_page.patch
> > * mm-streamline-cow-logic-in-do_swap_page.patch
> > * mm-huge_memory-streamline-cow-logic-in-do_huge_pmd_wp_page.patch
> > * mm-khugepaged-remove-reuse_swap_page-usage.patch
> > * mm-swapfile-remove-stale-reuse_swap_page.patch
> > * mm-huge_memory-remove-stale-page_trans_huge_mapcount.patch
> >
> > 4. Yang Shi sent his patch (the one we're replying to)
> >
> > 5. You picked his patch and dropped it again due to [2]
> >
> >
> > I'm wondering why 3 happened and why
> > https://www.ozlabs.org/~akpm/mmotm/series contains:
> >
> >
> > mm-optimize-do_wp_page-for-exclusive-pages-in-the-swapcache.patch
> > mm-optimize-do_wp_page-for-fresh-pages-in-local-lru-pagevecs.patch
> > mm-slightly-clarify-ksm-logic-in-do_swap_page.patch
> > mm-streamline-cow-logic-in-do_swap_page.patch
> > mm-huge_memory-streamline-cow-logic-in-do_huge_pmd_wp_page.patch
> > mm-khugepaged-remove-reuse_swap_page-usage.patch
> > mm-swapfile-remove-stale-reuse_swap_page.patch
> > mm-huge_memory-remove-stale-page_trans_huge_mapcount.patch
> > ...
> > #[merged]mm-huge_memory-remove-stale-locking-logic-from-__split_huge_pmd.patch
>
> OK, thanks. I guess it was me seeing 100% rejects when merging onto
> the folio changes then incorrectly deciding the patch was now in
> linux-next via some other tree.
>
> I restored it and fixed things up. Please check.
Thanks, Andrew. I think we could clean it up a little bit further.
>
>
> --- a/mm/huge_memory.c~mm-huge_memory-remove-stale-locking-logic-from-__split_huge_pmd
> +++ a/mm/huge_memory.c
> @@ -2133,8 +2133,6 @@ void __split_huge_pmd(struct vm_area_str
> {
> spinlock_t *ptl;
> struct mmu_notifier_range range;
> - bool do_unlock_folio = false;
> - pmd_t _pmd;
>
> mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
> address & HPAGE_PMD_MASK,
> @@ -2153,42 +2151,14 @@ void __split_huge_pmd(struct vm_area_str
> goto out;
> }
>
> -repeat:
> if (pmd_trans_huge(*pmd)) {
> - if (!folio) {
> + if (!folio)
> folio = page_folio(pmd_page(*pmd));
We could remove the "if (pmd_trans_huge(*pmd))" section since folio is
actually not used afterward at all.
> - /*
> - * An anonymous page must be locked, to ensure that a
> - * concurrent reuse_swap_page() sees stable mapcount;
> - * but reuse_swap_page() is not used on shmem or file,
> - * and page lock must not be taken when zap_pmd_range()
> - * calls __split_huge_pmd() while i_mmap_lock is held.
> - */
> - if (folio_test_anon(folio)) {
> - if (unlikely(!folio_trylock(folio))) {
> - folio_get(folio);
> - _pmd = *pmd;
> - spin_unlock(ptl);
> - folio_lock(folio);
> - spin_lock(ptl);
> - if (unlikely(!pmd_same(*pmd, _pmd))) {
> - folio_unlock(folio);
> - folio_put(folio);
> - folio = NULL;
> - goto repeat;
> - }
> - folio_put(folio);
> - }
> - do_unlock_folio = true;
> - }
> - }
> } else if (!(pmd_devmap(*pmd) || is_pmd_migration_entry(*pmd)))
> goto out;
> __split_huge_pmd_locked(vma, pmd, range.start, freeze);
With the above if removed, this could be changed to:
if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
is_pmd_migration_entry(*pmd))
__split_huge_pmd_locked(vma, pmd, range.start, freeze);
> out:
> spin_unlock(ptl);
> - if (do_unlock_folio)
> - folio_unlock(folio);
> /*
> * No need to double call mmu_notifier->invalidate_range() callback.
> * They are 3 cases to consider inside __split_huge_pmd_locked():
> _
>