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

To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Subject: Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in direct reclaim
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Thu, 14 Jul 2011 11:07:00 -0400
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Mel Gorman <mgorman@xxxxxxx>, Linux-MM <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, XFS <xfs@xxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Johannes Weiner <jweiner@xxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Minchan Kim <minchan.kim@xxxxxxxxx>
In-reply-to: <20110714134634.4a7a15c8.kamezawa.hiroyu@xxxxxxxxxxxxxx>
References: <1310567487-15367-1-git-send-email-mgorman@xxxxxxx> <1310567487-15367-2-git-send-email-mgorman@xxxxxxx> <20110714103801.83e10fdb.kamezawa.hiroyu@xxxxxxxxxxxxxx> <20110714044643.GA3203@xxxxxxxxxxxxx> <20110714134634.4a7a15c8.kamezawa.hiroyu@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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.

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