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Date: Thu, 14 Jul 2011 11:07:00 -0400
From: Christoph Hellwig <hch@infradead.org>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Christoph Hellwig <hch@infradead.org>, Mel Gorman <mgorman@suse.de>,
        Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
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        Wu Fengguang <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>,
        Rik van Riel <riel@redhat.com>, Minchan Kim <minchan.kim@gmail.com>
Subject: Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in
 direct reclaim
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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.