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Date:   Wed, 7 Jun 2023 15:13:46 -0400
From:   Peter Xu <peterx@redhat.com>
To:     David Stevens <stevensd@chromium.org>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Yang Shi <shy828301@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Jiaqi Yan <jiaqiyan@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/khugepaged: fix iteration in collapse_file
Message-ID: <ZIDW6qFAIwrYBOob@x1n>
References: <20230607053135.2087354-1-stevensd@google.com>
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On Wed, Jun 07, 2023 at 02:31:35PM +0900, David Stevens wrote:
> From: David Stevens <stevensd@chromium.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@chromium.org>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu