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Date:   Sun, 21 May 2023 16:55:11 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Yuanchu Xie <yuanchu@google.com>
cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>,
        Yang Shi <shy828301@gmail.com>,
        Zach O'Keefe <zokeefe@google.com>,
        Peter Xu <peterx@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] mm: pagemap: restrict pagewalk to the requested range
In-Reply-To: <20230515172608.3558391-1-yuanchu@google.com>
Message-ID: <a9fe6da9-f5da-d223-6ed0-fbcea5e3a618@google.com>
References: <20230515172608.3558391-1-yuanchu@google.com>
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On Tue, 16 May 2023, 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@google.com>

Acked-by: David Rientjes <rientjes@google.com>