diff mbox series

[v2,25/32] mm/gup: remove FOLL_SPLIT_PMD use of pmd_trans_unstable()

Message ID 59fd15dd-4d39-5ec-2043-1d5117f7f85@google.com
State New
Headers
Series mm: allow pte_offset_map[_lock]() to fail |

Commit Message

Hugh Dickins June 9, 2023, 1:39 a.m. UTC 
  There is now no reason for follow_pmd_mask()'s FOLL_SPLIT_PMD block to
distinguish huge_zero_page from a normal THP: follow_page_pte() handles
any instability, and here it's a good idea to replace any pmd_none(*pmd)
by a page table a.s.a.p, in the huge_zero_page case as for a normal THP;
and this removes an unnecessary possibility of -EBUSY failure.

(Hmm, couldn't the normal THP case have hit an unstably refaulted THP
before?  But there are only two, exceptional, users of FOLL_SPLIT_PMD.)

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/gup.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

Comments

Yang Shi June 9, 2023, 6:24 p.m. UTC | #1 
On Thu, Jun 8, 2023 at 6:40 PM Hugh Dickins <hughd@google.com> wrote:
>
> There is now no reason for follow_pmd_mask()'s FOLL_SPLIT_PMD block to
> distinguish huge_zero_page from a normal THP: follow_page_pte() handles
> any instability, and here it's a good idea to replace any pmd_none(*pmd)
> by a page table a.s.a.p, in the huge_zero_page case as for a normal THP;
> and this removes an unnecessary possibility of -EBUSY failure.
>
> (Hmm, couldn't the normal THP case have hit an unstably refaulted THP
> before?  But there are only two, exceptional, users of FOLL_SPLIT_PMD.)
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
>  mm/gup.c | 19 ++++---------------
>  1 file changed, 4 insertions(+), 15 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index bb67193c5460..4ad50a59897f 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -681,21 +681,10 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>                 return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
>         }
>         if (flags & FOLL_SPLIT_PMD) {
> -               int ret;
> -               page = pmd_page(*pmd);
> -               if (is_huge_zero_page(page)) {
> -                       spin_unlock(ptl);
> -                       ret = 0;
> -                       split_huge_pmd(vma, pmd, address);
> -                       if (pmd_trans_unstable(pmd))
> -                               ret = -EBUSY;
> -               } else {
> -                       spin_unlock(ptl);
> -                       split_huge_pmd(vma, pmd, address);
> -                       ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
> -               }
> -
> -               return ret ? ERR_PTR(ret) :
> +               spin_unlock(ptl);
> +               split_huge_pmd(vma, pmd, address);
> +               /* If pmd was left empty, stuff a page table in there quickly */
> +               return pte_alloc(mm, pmd) ? ERR_PTR(-ENOMEM) :
>                         follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
>         }
>         page = follow_trans_huge_pmd(vma, address, pmd, flags);
> --
> 2.35.3
>
diff mbox series

Patch

diff --git a/mm/gup.c b/mm/gup.c
index bb67193c5460..4ad50a59897f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -681,21 +681,10 @@  static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
 	}
 	if (flags & FOLL_SPLIT_PMD) {
-		int ret;
-		page = pmd_page(*pmd);
-		if (is_huge_zero_page(page)) {
-			spin_unlock(ptl);
-			ret = 0;
-			split_huge_pmd(vma, pmd, address);
-			if (pmd_trans_unstable(pmd))
-				ret = -EBUSY;
-		} else {
-			spin_unlock(ptl);
-			split_huge_pmd(vma, pmd, address);
-			ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
-		}
-
-		return ret ? ERR_PTR(ret) :
+		spin_unlock(ptl);
+		split_huge_pmd(vma, pmd, address);
+		/* If pmd was left empty, stuff a page table in there quickly */
+		return pte_alloc(mm, pmd) ? ERR_PTR(-ENOMEM) :
 			follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
 	}
 	page = follow_trans_huge_pmd(vma, address, pmd, flags);