linux-kernel - Re: [PATCH v2 08/28] gup: Handle page split race more efficiently

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Date:   Mon, 10 Jan 2022 00:31:10 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        William Kucharski <william.kucharski@...cle.com>,
        linux-kernel@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH v2 08/28] gup: Handle page split race more efficiently

On Mon, Jan 10, 2022 at 04:23:46AM +0000, Matthew Wilcox (Oracle) wrote:
> If we hit the page split race, the current code returns NULL which will
> presumably trigger a retry under the mmap_lock.  This isn't necessary;
> we can just retry the compound_head() lookup.  This is a very minor
> optimisation of an unlikely path, but conceptually it matches (eg)
> the page cache RCU-protected lookup.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>

Looks good,

Reviewed-by: Christoph Hellwig <hch@....de>