linux-kernel - Re: [PATCH] mm: pagemap: restrict pagewalk to the requested range

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Message-ID: <ZGKsxlVpcTRUFT6N@x1n>
Date:   Mon, 15 May 2023 18:05:58 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Yuanchu Xie <yuanchu@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yang Shi <shy828301@...il.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: pagemap: restrict pagewalk to the requested range

On Tue, May 16, 2023 at 01:26:08AM +0800, Yuanchu Xie wrote:
> The pagewalk in pagemap_read reads one PTE past the end of the requested
> range, and stops when the buffer runs out of space. While it produces
> the right result, the extra read is unnecessary and less performant.
> 
> I timed the following command before and after this patch:
> 	dd count=100000 if=/proc/self/pagemap of=/dev/null
> The results are consistently within 0.001s across 5 runs.
> 
> Before:
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB) copied, 0.0763159 s, 671 MB/s
> 
> real    0m0.078s
> user    0m0.012s
> sys     0m0.065s
> 
> After:
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB) copied, 0.0487928 s, 1.0 GB/s
> 
> real    0m0.050s
> user    0m0.011s
> sys     0m0.039s
> 
> Signed-off-by: Yuanchu Xie <yuanchu@...gle.com>

Acked-by: Peter Xu <peterx@...hat.com>

-- 
Peter Xu