[PATCH 1/3] mm: Allow anon exclusive check over hugetlb tail pages

[peterx]

From: Peter Xu <peterx@xxxxxxxxxx>

PageAnonExclusive() used to forbid tail pages for hugetlbfs, as that used
to be called mostly in hugetlb specific paths and the head page was
guaranteed.

As we move forward towards merging hugetlb paths into generic mm, we may
start to pass in tail hugetlb pages (when with cont-pte/cont-pmd huge
pages) for such check.  Allow it to properly fetch the head, in which case
the anon-exclusiveness of the head will always represents the tail page.

There's already a sign of it when we look at the fast-gup which already
contain the hugetlb processing altogether: we used to have a specific
commit 5805192c7b72 ("mm/gup: handle cont-PTE hugetlb pages correctly in
gup_must_unshare() via GUP-fast") covering that area.  Now with this more
generic change, that can also go away.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 include/linux/page-flags.h |  8 +++++++-
 mm/internal.h              | 10 ----------
 2 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 888353c209c0..225357f48a79 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1095,7 +1095,13 @@ PAGEFLAG(Isolated, isolated, PF_ANY);
 static __always_inline int PageAnonExclusive(const struct page *page)
 {
 	VM_BUG_ON_PGFLAGS(!PageAnon(page), page);
-	VM_BUG_ON_PGFLAGS(PageHuge(page) && !PageHead(page), page);
+	/*
+	 * Allow the anon-exclusive check to work on hugetlb tail pages.
+	 * Here hugetlb pages will always guarantee the anon-exclusiveness
+	 * of the head page represents the tail pages.
+	 */
+	if (PageHuge(page) && !PageHead(page))
+		page = compound_head(page);
 	return test_bit(PG_anon_exclusive, &PF_ANY(page, 1)->flags);
 }
 
diff --git a/mm/internal.h b/mm/internal.h
index 9512de7398d5..87f6e4fd56a5 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1259,16 +1259,6 @@ static inline bool gup_must_unshare(struct vm_area_struct *vma,
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
 		smp_rmb();
 
-	/*
-	 * During GUP-fast we might not get called on the head page for a
-	 * hugetlb page that is mapped using cont-PTE, because GUP-fast does
-	 * not work with the abstracted hugetlb PTEs that always point at the
-	 * head page. For hugetlb, PageAnonExclusive only applies on the head
-	 * page (as it cannot be partially COW-shared), so lookup the head page.
-	 */
-	if (unlikely(!PageHead(page) && PageHuge(page)))
-		page = compound_head(page);
-
 	/*
 	 * Note that PageKsm() pages cannot be exclusive, and consequently,
 	 * cannot get pinned.
-- 
2.44.0