Re: [PATCH] mm: thp: don't have to lock page anymore when splitting PMD

[Andrew Morton]

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.


--- 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():
_