linux-kernel - Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20170802221359.GA20666@linux.intel.com>
Date:   Wed, 2 Aug 2017 16:13:59 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, "karam . lee" <karam.lee@....com>,
        Minchan Kim <minchan@...nel.org>,
        Jerome Marchand <jmarchan@...hat.com>,
        Nitin Gupta <ngupta@...are.org>, seungho1.park@....com,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>,
        Vishal Verma <vishal.l.verma@...el.com>,
        linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt

On Fri, Jul 28, 2017 at 10:31:43AM -0700, Matthew Wilcox wrote:
> On Fri, Jul 28, 2017 at 10:56:01AM -0600, Ross Zwisler wrote:
> > Dan Williams and Christoph Hellwig have recently expressed doubt about
> > whether the rw_page() interface made sense for synchronous memory drivers
> > [1][2].  It's unclear whether this interface has any performance benefit
> > for these drivers, but as we continue to fix bugs it is clear that it does
> > have a maintenance burden.  This series removes the rw_page()
> > implementations in brd, pmem and btt to relieve this burden.
> 
> Why don't you measure whether it has performance benefits?  I don't
> understand why zram would see performance benefits and not other drivers.
> If it's going to be removed, then the whole interface should be removed,
> not just have the implementations removed from some drivers.

Okay, I've run a bunch of performance tests with the PMEM and with BTT entry
points for rw_pages() in a swap workload, and in all cases I do see an
improvement over the code when rw_pages() is removed.  Here are the results
from my random lab box:

  Average latency of swap_writepage()
+------+------------+---------+-------------+
|      | no rw_page | rw_page | Improvement |
+-------------------------------------------+
| PMEM |  5.0 us    |  4.7 us |     6%      |
+-------------------------------------------+
|  BTT |  6.8 us    |  6.1 us |    10%      |
+------+------------+---------+-------------+

  Average latency of swap_readpage()
+------+------------+---------+-------------+
|      | no rw_page | rw_page | Improvement |
+-------------------------------------------+
| PMEM |  3.3 us    |  2.9 us |    12%      |
+-------------------------------------------+
|  BTT |  3.7 us    |  3.4 us |     8%      |
+------+------------+---------+-------------+

The workload was pmbench, a memory benchmark, run on a system where I had
severely restricted the amount of memory in the system with the 'mem' kernel
command line parameter.  The benchmark was set up to test more memory than I
allowed the OS to have so it spilled over into swap.

The PMEM or BTT device was set up as my swap device, and during the test I got
a few hundred thousand samples of each of swap_writepage() and
swap_writepage().  The PMEM/BTT device was just memory reserved with the
memmap kernel command line parameter.

Thanks, Matthew, for asking for performance data.  It looks like removing this
code would have been a mistake.