[PATCH] mm: Remove bogus VM_BUG_ON

From: Matthew Wilcox (Oracle)
Date: Wed Aug 18 2021 - 10:51:30 EST

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.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 5c211ba29deb ("mm: add and use find_lock_entries")
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
mm/filemap.c | 1 -
1 file changed, 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index d1458ecf2f51..34de0b14aaa9 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2033,17 +2033,16 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
XA_STATE(xas, &mapping->i_pages, start);
struct page *page;

while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
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))
goto put;
if (page->mapping != mapping || PageWriteback(page))
goto unlock;
VM_BUG_ON_PAGE(!thp_contains(page, xas.xa_index),