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

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Date:	Wed, 16 Jun 2010 01:11:33 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	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>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>, axboe@...nel.dk
Subject: Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

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.
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