linux-kernel - Re: [patch 1/5] mm: exclude reserved pages from dirtyable memory

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: <20111129162014.aa290174.akpm@linux-foundation.org>
Date:	Tue, 29 Nov 2011 16:20:14 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Michal Hocko <mhocko@...e.cz>,
	Christoph Hellwig <hch@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Shaohua Li <shaohua.li@...el.com>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/5] mm: exclude reserved pages from dirtyable memory

On Wed, 23 Nov 2011 14:34:14 +0100
Johannes Weiner <hannes@...xchg.org> wrote:

> From: Johannes Weiner <jweiner@...hat.com>
> 
> The amount of dirtyable pages should not include the full number of
> free pages: there is a number of reserved pages that the page
> allocator and kswapd always try to keep free.
> 
> The closer (reclaimable pages - dirty pages) is to the number of
> reserved pages, the more likely it becomes for reclaim to run into
> dirty pages:
> 
>        +----------+ ---
>        |   anon   |  |
>        +----------+  |
>        |          |  |
>        |          |  -- dirty limit new    -- flusher new
>        |   file   |  |                     |
>        |          |  |                     |
>        |          |  -- dirty limit old    -- flusher old
>        |          |                        |
>        +----------+                       --- reclaim
>        | reserved |
>        +----------+
>        |  kernel  |
>        +----------+
> 
> This patch introduces a per-zone dirty reserve that takes both the
> lowmem reserve as well as the high watermark of the zone into account,
> and a global sum of those per-zone values that is subtracted from the
> global amount of dirtyable pages.  The lowmem reserve is unavailable
> to page cache allocations and kswapd tries to keep the high watermark
> free.  We don't want to end up in a situation where reclaim has to
> clean pages in order to balance zones.
> 
> Not treating reserved pages as dirtyable on a global level is only a
> conceptual fix.  In reality, dirty pages are not distributed equally
> across zones and reclaim runs into dirty pages on a regular basis.
> 
> But it is important to get this right before tackling the problem on a
> per-zone level, where the distance between reclaim and the dirty pages
> is mostly much smaller in absolute numbers.
> 
> ...
>
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -327,7 +327,8 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
>  			&NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
>  
>  		x += zone_page_state(z, NR_FREE_PAGES) +
> -		     zone_reclaimable_pages(z);
> +		     zone_reclaimable_pages(z) -
> +		     zone->dirty_balance_reserve;

Doesn't compile.  s/zone/z/.

Which makes me suspect it wasn't tested on a highmem box.  This is
rather worrisome, as highmem machines tend to have acute and unique
zone balancing issues.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/