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Message-ID: <20170630190556.GB27371@linux.intel.com> Date: Fri, 30 Jun 2017 13:05:56 -0600 From: Ross Zwisler <ross.zwisler@...ux.intel.com> To: Ross Zwisler <ross.zwisler@...ux.intel.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, "Darrick J. Wong" <darrick.wong@...cle.com>, Theodore Ts'o <tytso@....edu>, Alexander Viro <viro@...iv.linux.org.uk>, Andreas Dilger <adilger.kernel@...ger.ca>, Christoph Hellwig <hch@....de>, Dan Williams <dan.j.williams@...el.com>, Dave Hansen <dave.hansen@...el.com>, Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.cz>, Jonathan Corbet <corbet@....net>, Matthew Wilcox <mawilcox@...rosoft.com>, Steven Rostedt <rostedt@...dmis.org>, linux-doc@...r.kernel.org, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-nvdimm@...ts.01.org, linux-xfs@...r.kernel.org Subject: Re: [PATCH v3 0/5] DAX common 4k zero page On Wed, Jun 28, 2017 at 04:01:47PM -0600, Ross Zwisler wrote: > When servicing mmap() reads from file holes the current DAX code allocates > a page cache page of all zeroes and places the struct page pointer in the > mapping->page_tree radix tree. This has three major drawbacks: > > 1) It consumes memory unnecessarily. For every 4k page that is read via a > DAX mmap() over a hole, we allocate a new page cache page. This means that > if you read 1GiB worth of pages, you end up using 1GiB of zeroed memory. > > 2) It is slower than using a common zero page because each page fault has > more work to do. Instead of just inserting a common zero page we have to > allocate a page cache page, zero it, and then insert it. > > 3) The fact that we had to check for both DAX exceptional entries and for > page cache pages in the radix tree made the DAX code more complex. > > This series solves these issues by following the lead of the DAX PMD code > and using a common 4k zero page instead. This reduces memory usage and > decreases latencies for some workloads, and it simplifies the DAX code, > removing over 100 lines in total. > > Andrew, I'm still hoping to get this merged for v4.13 if possible. I I have > addressed all of Jan's feedback, but he is on vacation for the next few > weeks so he may not be able to give me Reviewed-by tags. I think this > series is relatively low risk with clear benefits, and I think we should be > able to address any issues that come up during the v4.13 RC series. > > This series has passed my targeted testing and a full xfstests run on both > XFS and ext4. This series has also passed the automated 0-day kernel builds in 168 configs.
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