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

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Message-ID: <20160901085111.GC8119@techsingularity.net>
Date:   Thu, 1 Sep 2016 09:51:11 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "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, Aug 31, 2016 at 02:30:31PM -0700, 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?
> 

It'll stop silent breakage so

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

Whoever hits it will need to take similar steps we had to with page->flags
by making some 64-bit only, removing flags or inferring the flag values
from other sources.

-- 
Mel Gorman
SUSE Labs