[023/227] mm/gup: follow_pfn_pte(): -EEXIST cleanup

Commit Message

Andrew Morton March 22, 2022, 9:39 p.m. UTC

From: John Hubbard <[e-mail]>
Subject: mm/gup: follow_pfn_pte(): -EEXIST cleanup

Remove a quirky special case from follow_pfn_pte(), and adjust its callers
to match.  Caller changes include:

__get_user_pages(): Regardless of any FOLL_* flags, get_user_pages() and
its variants should handle PFN-only entries by stopping early, if the
caller expected **pages to be filled in.  This makes for a more reliable
API, as compared to the previous approach of skipping over such entries
(and thus leaving them silently unwritten).

move_pages(): squash the -EEXIST error return from follow_page() into
-EFAULT, because -EFAULT is listed in the man page, whereas -EEXIST is
not.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: John Hubbard <[e-mail]>
Suggested-by: Jason Gunthorpe <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Lukas Bulwahn <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Claudio Imbrenda <[e-mail]>
Cc: Alex Williamson <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/gup.c     |   13 ++++++++-----
 mm/migrate.c |    7 +++++++
 2 files changed, 15 insertions(+), 5 deletions(-)

Patch

--- a/mm/gup.c~mm-gup-follow_pfn_pte-eexist-cleanup
+++ a/mm/gup.c
@@ -464,10 +464,6 @@  static struct page *no_page_table(struct
 static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
 		pte_t *pte, unsigned int flags)
 {
-	/* No page to get reference */
-	if (flags & (FOLL_GET | FOLL_PIN))
-		return -EFAULT;
-
 	if (flags & FOLL_TOUCH) {
 		pte_t entry = *pte;
 
@@ -1205,8 +1201,15 @@  retry:
 		} else if (PTR_ERR(page) == -EEXIST) {
 			/*
 			 * Proper page table entry exists, but no corresponding
-			 * struct page.
+			 * struct page. If the caller expects **pages to be
+			 * filled in, bail out now, because that can't be done
+			 * for this page.
 			 */
+			if (pages) {
+				ret = PTR_ERR(page);
+				goto out;
+			}
+
 			goto next_page;
 		} else if (IS_ERR(page)) {
 			ret = PTR_ERR(page);
--- a/mm/migrate.c~mm-gup-follow_pfn_pte-eexist-cleanup
+++ a/mm/migrate.c
@@ -1762,6 +1762,13 @@  static int do_pages_move(struct mm_struc
 		}
 
 		/*
+		 * The move_pages() man page does not have an -EEXIST choice, so
+		 * use -EFAULT instead.
+		 */
+		if (err == -EEXIST)
+			err = -EFAULT;
+
+		/*
 		 * If the page is already on the target node (!err), store the
 		 * node, otherwise, store the err.
 		 */