linux-kernel - [PATCH] mm: Remove bogus VM_BUG

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Date:   Wed, 18 Aug 2021 15:49:32 +0100
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        stable@...r.kernel.org
Subject: [PATCH] mm: 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.

Cc: stable@...r.kernel.org
Fixes: 5c211ba29deb ("mm: add and use find_lock_entries")
Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 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;
 
 	rcu_read_lock();
 	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),
 					page);
-- 
2.30.2