linux-kernel - [PATCH v3 2/4] mm/gup: clean up follow_pfn

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Date:   Thu, 3 Feb 2022 01:32:30 -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>,
        Jan Kara <jack@...e.cz>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "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 v3 2/4] mm/gup: clean up follow_pfn_pte() slightly

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).

Cc: Peter Xu <peterx@...hat.com>
Cc: Lukas Bulwahn <lukas.bulwahn@...il.com>
Suggested-by: Jason Gunthorpe <jgg@...dia.com>
Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
Signed-off-by: John Hubbard <jhubbard@...dia.com>
---
 mm/gup.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 65575ae3602f..cad3f28492e3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -439,10 +439,6 @@ 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;
 
@@ -1180,8 +1176,13 @@ 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)
+				goto out;
+
 			goto next_page;
 		} else if (IS_ERR(page)) {
 			ret = PTR_ERR(page);
-- 
2.35.1