linux-kernel - Re: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in swap cache

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Date:   Thu, 1 Sep 2016 11:13:48 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        "Huang, Ying" <ying.huang@...el.com>, tim.c.chen@...el.com,
        dave.hansen@...el.com, andi.kleen@...el.com, aaron.lu@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
        Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in
 swap cache

On Wed 31-08-16 14:30:31, Andrew Morton wrote:
> On Wed, 31 Aug 2016 10:14:59 +0100 Mel Gorman <mgorman@...hsingularity.net> wrote:
> 
> > >    2506952 __  2%     +28.1%    3212076 __  7%  vm-scalability.throughput
> > >    1207402 __  7%     +22.3%    1476578 __  6%  vmstat.swap.so
> > >      10.86 __ 12%     -23.4%       8.31 __ 16%  perf-profile.cycles-pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_swap.shrink_page_list
> > >      10.82 __ 13%     -33.1%       7.24 __ 14%  perf-profile.cycles-pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_inactive_list.shrink_zone_memcg
> > >      10.36 __ 11%    -100.0%       0.00 __ -1%  perf-profile.cycles-pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page.__swap_writepage.swap_writepage
> > >      10.52 __ 12%    -100.0%       0.00 __ -1%  perf-profile.cycles-pp._raw_spin_lock_irqsave.test_clear_page_writeback.end_page_writeback.page_endio.pmem_rw_page
> > > 
> > 
> > I didn't see anything wrong with the patch but it's worth highlighting
> > that this hunk means we are now out of GFP bits.
> 
> Well ugh.  What are we to do about that?

Can we simply give these AS_ flags their own word in mapping rather than
squash them together with gfp flags and impose the restriction on the
number of gfp flags. There was some demand for new gfp flags already and
mapping flags were in the way.
-- 
Michal Hocko
SUSE Labs