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SubjectRe: [PATCH] mm: thp: don't have to lock page anymore when splitting PMD
On Mon, 7 Mar 2022 09:24:58 +0100 David Hildenbrand <david@redhat.com> wrote:

> On 07.03.22 03:07, Andrew Morton wrote:
> > On Fri, 4 Mar 2022 19:50:08 +0100 David Hildenbrand <david@redhat.com> 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@mail.gmail.com
> >
>
> 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.


--- 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));
- /*
- * 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);
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():
_
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