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

[Miaohe Lin]

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():
>