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From:   John Hubbard <jhubbard@nvidia.com>
To:     Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>
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        LKML <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>
Subject: [PATCH v2 2/4] mm/gup: clean up follow_pfn_pte() slightly
Date:   Tue, 1 Feb 2022 02:11:06 -0800
Message-ID: <20220201101108.306062-3-jhubbard@nvidia.com>
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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@redhat.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 65575ae3602f..8633bca12eab 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 +1176,14 @@ 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) {
+				page = ERR_PTR(-EFAULT);
+				goto out;
+			}
 			goto next_page;
 		} else if (IS_ERR(page)) {
 			ret = PTR_ERR(page);
-- 
2.35.1