mm/filemap.c: remove bogus VM_BUG_ON

It is not safe to check page->index without holding the page lock.  It can
be changed if the page is moved between the swap cache and the page cache
for a shmem file, for example.  There is a VM_BUG_ON below which checks
page->index is correct after taking the page lock.

Link: https://lkml.kernel.org/r/20210818144932.940640-1-willy@infradead.org
Fixes: 5c211ba29deb ("mm: add and use find_lock_entries")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: <syzbot+c87be4f669d920c76330@syzkaller.appspotmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

diff --git a/mm/filemap.c b/mm/filemap.c
index d1458ecf2f51ec2100eae25e3c53801e96acc0e5..34de0b14aaa90fae1460616ed166abd141dc88cc 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2038,7 +2038,6 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
                if (!xa_is_value(page)) {
                        if (page->index < start)
                                goto put;
-                       VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
                        if (page->index + thp_nr_pages(page) - 1 > end)
                                goto put;
                        if (!trylock_page(page))