From:   David Stevens <stevensd@chromium.org>
To:     linux-mm@kvack.org, Peter Xu <peterx@redhat.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Yang Shi <shy828301@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>, linux-kernel@vger.kernel.org,
        David Stevens <stevensd@chromium.org>
Subject: [PATCH 1/2] mm/khugepaged: set THP as uptodate earlier for shmem
Date:   Tue, 14 Feb 2023 16:57:09 +0900
Message-Id: <20230214075710.2401855-1-stevensd@google.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

From: David Stevens <stevensd@chromium.org>

In collapse_file, mark the THP as up-to-date before inserting it into
the page cache. This fixes a race where folio_seek_hole_data would
mistake the THP for an fallocated but unwritten page. This race is
visible to userspace via data temporarily disappearing from
SEEK_DATA/SEEK_HOLE, which can cause data loss for applications that use
lseek to efficiently snapshot sparse shmem.

Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: David Stevens <stevensd@chromium.org>
---
 mm/khugepaged.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 79be13133322..b648f1053d95 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1779,10 +1779,13 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	hpage->mapping = mapping;
 
 	/*
-	 * At this point the hpage is locked and not up-to-date.
-	 * It's safe to insert it into the page cache, because nobody would
-	 * be able to map it or use it in another way until we unlock it.
+	 * Mark hpage as up-to-date before inserting it into the page cache to
+	 * prevent it from being mistaken for an fallocated but unwritten page.
+	 * Inserting the unfinished hpage into the page cache is safe because
+	 * it is locked, so nobody can map it or use it in another way until we
+	 * unlock it.
 	 */
+	SetPageUptodate(hpage);
 
 	xas_set(&xas, start);
 	for (index = start; index < end; index++) {
-- 
2.39.1.581.gbfd45094c4-goog