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

Hugh Dickins
2023-06-09T01:40:07Z

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(-)

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