linux-kernel - [PATCH v4 2/5] mm/gup: follow_pfn

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Date:   Thu, 3 Feb 2022 18:00:07 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>
CC:     David Hildenbrand <david@...hat.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>
Subject: [PATCH v4 2/5] 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.

Cc: Peter Xu <peterx@...hat.com>
Cc: Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc: Jan Kara <jack@...e.cz>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Claudio Imbrenda <imbrenda@...ux.ibm.com>
Suggested-by: Jason Gunthorpe <jgg@...dia.com>
Signed-off-by: John Hubbard <jhubbard@...dia.com>
---
 mm/gup.c     | 13 ++++++++-----
 mm/migrate.c |  7 +++++++
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 80229ecf0114..2df0d0103c43 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -464,10 +464,6 @@ static struct page *no_page_table(struct vm_area_struct *vma,
 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 @@ static long __get_user_pages(struct mm_struct *mm,
 		} 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);
diff --git a/mm/migrate.c b/mm/migrate.c
index c7da064b4781..be0d5ae36dc1 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1761,6 +1761,13 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 			continue;
 		}
 
+		/*
+		 * 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.
-- 
2.35.1