[PATCH v2 12/15] mm/gup: disallow follow_page(FOLL_PIN)

[David Hildenbrand <david@redhat.com>]

We want to change the way we handle R/O pins on anonymous pages that
might be shared: if we detect a possibly shared anonymous page --
mapped R/O and not !PageAnonExclusive() -- we want to trigger unsharing
via a page fault, resulting in an exclusive anonymous page that can be
pinned reliably without getting replaced via COW on the next write
fault.

However, the required page fault will be problematic for follow_page():
in contrast to ordinary GUP, follow_page() doesn't trigger faults
internally. So we would have to end up failing a R/O pin via
follow_page(), although there is something mapped R/O into the page
table, which might be rather surprising.

We don't seem to have follow_page(FOLL_PIN) users, and it's a purely
internal MM function. Let's just make our life easier and the semantics of
follow_page() clearer by just disallowing FOLL_PIN for follow_page()
completely.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/gup.c     | 3 +++
 mm/hugetlb.c | 8 +++++---
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 7bc1ba9ce440..ae1aa2e93b0f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -875,6 +875,9 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 	if (vma_is_secretmem(vma))
 		return NULL;
 
+	if (foll_flags & FOLL_PIN)
+		return NULL;
+
 	page = follow_page_mask(vma, address, foll_flags, &ctx);
 	if (ctx.pgmap)
 		put_dev_pagemap(ctx.pgmap);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c0e19ea5ebb5..640364a11336 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6692,9 +6692,11 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
 	spinlock_t *ptl;
 	pte_t pte;
 
-	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
-	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
-			 (FOLL_PIN | FOLL_GET)))
+	/*
+	 * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
+	 * follow_hugetlb_page().
+	 */
+	if (WARN_ON_ONCE(flags & FOLL_PIN))
 		return NULL;
 
 retry:
-- 
2.35.1