mm/filemap.c: remove bogus VM_BUG_ON

mainline inclusion
from mainline-v5.16-rc1
commit d417b49f
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6110W
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d417b49fff3e2f21043c834841e8623a6098741d

--------------------------------

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: 5c211ba2 ("mm: add and use find_lock_entries")
Signed-off-by: NMatthew 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: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NMa Wupeng <mawupeng1@huawei.com>
Reviewed-by: Ntong tiangen <tongtiangen@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Signed-off-by: NJialin Zhang <zhangjialin11@huawei.com>

mm/filemap.c

@@ -1956,7 +1956,6 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
 				next_idx = page->index + thp_nr_pages(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))