linux-kernel - Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in direct reclaim

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Message-Id: <20110715085520.23feca2d.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 15 Jul 2011 08:55:20 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Mel Gorman <mgorman@...e.de>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, XFS <xfs@....sgi.com>,
	Dave Chinner <david@...morbit.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in
 direct reclaim

On Thu, 14 Jul 2011 11:07:00 -0400
Christoph Hellwig <hch@...radead.org> wrote:

> On Thu, Jul 14, 2011 at 01:46:34PM +0900, KAMEZAWA Hiroyuki wrote:
> > > XFS and btrfs already disable writeback from memcg context, as does ext4
> > > for the typical non-overwrite workloads, and none has fallen apart.
> > > 
> > > In fact there's no way we can enable them as the memcg calling contexts
> > > tend to have massive stack usage.
> > > 
> > 
> > Hmm, XFS/btrfs adds pages to radix-tree in deep stack ?
> 
> We're using a fairly deep stack in normal buffered read/write,
> wich is almost 100% common code.  It's not just the long callchain
> (see below), but also that we put the unneeded kiocb and a vector
> of I/O vects on the stack:
> 
> vfs_writev
> do_readv_writev
> do_sync_write
> generic_file_aio_write
> __generic_file_aio_write
> generic_file_buffered_write
> generic_perform_write
> block_write_begin
> grab_cache_page_write_begin
> add_to_page_cache_lru
> add_to_page_cache
> add_to_page_cache_locked
> mem_cgroup_cache_charge
> 
> this might additionally come from in-kernel callers like nfsd,
> which has even more stack space used.  And at this point we only
> enter the memcg/reclaim code, which last time I had a stack trace
> ate up another about 3k of stack space.
> 

Hmm. I'll prepare 2 functions for memcg 
  1. asynchronous memory reclaim as kswapd does.
  2. dirty_ratio

please remove ->writepage 1st. It may break memcg but it happens sometimes.
We'll do fix.

Thanks,
-Kame

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