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Date:   Tue, 15 Feb 2022 20:12:13 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 04/10] mm/truncate: Replace page_mapped() call in
 invalidate_inode_page()
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On Mon, Feb 14, 2022 at 11:19:22PM -0800, Christoph Hellwig wrote:
> On Mon, Feb 14, 2022 at 08:00:11PM +0000, Matthew Wilcox (Oracle) wrote:
> > folio_mapped() is expensive because it has to check each page's mapcount
> > field.  A cheaper check is whether there are any extra references to
> > the page, other than the one we own and the ones held by the page cache.
> > The call to remove_mapping() will fail in any case if it cannot freeze
> > the refcount, but failing here avoids cycling the i_pages spinlock.
> 
> I wonder if something like this should also be in a comment near
> the check in the code.

        /* The refcount will be elevated if any page in the folio is mapped */

is what I've added for now.