linux-kernel - [PATCH] mm/khugepaged: fix iteration in collapse

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Date:   Wed,  7 Jun 2023 14:31:35 +0900
From:   David Stevens <stevensd@...omium.org>
To:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>
Cc:     Peter Xu <peterx@...hat.com>, Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Yang Shi <shy828301@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jiaqi Yan <jiaqiyan@...gle.com>, linux-kernel@...r.kernel.org,
        David Stevens <stevensd@...omium.org>
Subject: [PATCH] mm/khugepaged: fix iteration in collapse_file

From: David Stevens <stevensd@...omium.org>

Remove an unnecessary call to xas_set(index) when iterating over the
target range in collapse_file. The extra call to xas_set reset the xas
cursor to the top of the tree, causing the xas_next call on the next
iteration to walk the tree to index instead of advancing to index+1.
This returned the same page again, which would cause collapse_file to
fail because the page is already locked.

This bug was hidden when CONFIG_DEBUG_VM was set. When that config was
used, the xas_load in a subsequent VM_BUG_ON assert would walk xas from
the top of the tree to index, causing the xas_next call on the next loop
iteration to advance the cursor as expected.

Fixes: a2e17cc2efc7 ("mm/khugepaged: maintain page cache uptodate flag")
Signed-off-by: David Stevens <stevensd@...omium.org>
---
 mm/khugepaged.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6b9d39d65b73..2d0d58fb4e7f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2070,7 +2070,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 					TTU_IGNORE_MLOCK | TTU_BATCH_FLUSH);

 		xas_lock_irq(&xas);
-		xas_set(&xas, index);

 		VM_BUG_ON_PAGE(page != xas_load(&xas), page);

-- 
2.41.0.rc2.161.g9c6817b8e7-goog