linux-kernel - Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

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Date:	Wed, 16 Jun 2010 12:51:39 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Christoph Hellwig <hch@...radead.org>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

On 2010-06-16 07:11, Christoph Hellwig wrote:
> On Wed, Jun 16, 2010 at 11:20:24AM +0900, KAMEZAWA Hiroyuki wrote:
>> BTW, copy_from_user/copy_to_user is _real_ problem, I'm afraid following
>> much more than memcg.
>>
>> handle_mm_fault()
>> -> handle_pte_fault()
>> -> do_wp_page()
>> -> balance_dirty_page_rate_limited()
>> -> balance_dirty_pages()
>> -> writeback_inodes_wbc()
>> -> writeback_inodes_wb()
>> -> writeback_sb_inodes()
>> -> writeback_single_inode()
>> -> do_writepages()
>> -> generic_write_pages()
>> -> write_cache_pages()   // use on-stack pagevec.
>> -> writepage()
> 
> Yes, this is a massive issue.  Strangely enough I just wondered about
> this callstack as balance_dirty_pages is the only place calling into the
> per-bdi/sb writeback code directly instead of offloading it to the
> flusher threads.  It's something that should be fixed rather quickly
> IMHO.  write_cache_pages and other bits of this writeback code can use
> quite large amounts of stack.

I've had the same thought as well, bdp() should just signal a writeback
instead. Much cleaner than doing cleaning from that point.

-- 
Jens Axboe

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