On 2022/3/4 6:20, Yang Shi wrote:
> The commit c444eb564fb1 ("mm: thp: make the THP mapcount atomic against
> __split_huge_pmd_locked()") locked the page for PMD split to make
> mapcount stable for reuse_swap_page(), then commit 1c2f67308af4 ("mm:
> thp: fix MADV_REMOVE deadlock on shmem THP") reduce the scope to
> anonymous page only.
>
> However COW has not used mapcount to determine if the page is shared or
> not anymore due to the COW fixes [1] from David Hildenbrand and the
> reuse_swap_page() was removed as well. So PMD split doesn't have to
> lock the page anymore. This patch basically reverted the above two
> commits.
>
Sounds reasonable. Since mapcount is not used to determine if we need to COW
the page, PMD split doesn't have to lock the page anymore.
> [1] https://lore.kernel.org/linux-mm/20220131162940.210846-1-david@xxxxxxxxxx/
>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> ---
> mm/huge_memory.c | 44 +++++---------------------------------------
> 1 file changed, 5 insertions(+), 39 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b49e1a11df2e..daaa698bd273 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2134,8 +2134,6 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> {
> 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,
> @@ -2148,48 +2146,16 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> * pmd against. Otherwise we can end up replacing wrong folio.
> */
> VM_BUG_ON(freeze && !folio);
> - if (folio) {
> - VM_WARN_ON_ONCE(!folio_test_locked(folio));
> - if (folio != page_folio(pmd_page(*pmd)))
> - goto out;
> - }
> + if (folio && folio != page_folio(pmd_page(*pmd)))
> + goto out;
>
> -repeat:
> - if (pmd_trans_huge(*pmd)) {
> - 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)))
> + if (!(pmd_devmap(*pmd) || is_pmd_migration_entry(*pmd)))
> goto out;
> +
> __split_huge_pmd_locked(vma, pmd, range.start, freeze);
IUUC, here should be something like below:
if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || is_pmd_migration_entry(*pmd))
__split_huge_pmd_locked(vma, pmd, range.start, freeze);
i.e. the pmd_trans_huge case is missing in the above change. Or am I miss something ?
Thanks for the patch. This really simplify the code and avoid the unneeded overhead.
> 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():
>
