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

[Hugh Dickins]

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@xxxxxxxxxx>
---
 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