linux-kernel - Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

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Date:	Mon, 14 Jun 2010 21:37:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Dave Chinner <david@...morbit.com>, Mel Gorman <mel@....ul.ie>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

On Tue, 15 Jun 2010 00:08:14 -0400 Rik van Riel <riel@...hat.com> wrote:

> On 06/14/2010 09:45 PM, Andrew Morton wrote:
> > On Mon, 14 Jun 2010 21:16:29 -0400 Rik van Riel<riel@...hat.com>  wrote:
> >
> >> Would it be hard to add a "please flush this file"
> >> way to call the filesystem flushing threads?
> >
> > Passing the igrab()bed inode into the flusher threads would fix the
> > iput_final() problems, as long as the alloc_pages() caller never blocks
> > indefinitely waiting for the work which the flusher threads are doing.
> >
> > Otherwise we get (very hard-to-hit) deadlocks where the alloc_pages()
> > caller holds VFS locks and is waiting for the flusher threads while all
> > the flusher threads are stuck under iput_final() waiting for those VFS
> > locks.
> >
> > That's fixable by not using igrab()/iput().  You can use lock_page() to
> > pin the address_space.  Pass the address of the locked page across to
> > the flusher threads so they don't try to lock it a second time, or just
> > use trylocking on that writeback path or whatever.
> 
> Any thread that does not have __GFP_FS set in its gfp_mask
> cannot wait for the flusher to complete. This is regardless
> of the mechanism used to kick the flusher.

mm...  kinda.  A bare order-zero __GFP_WAIT allocation can still wait
forever, afaict.

> Then again, those threads cannot call ->writepage today
> either, so we should be fine keeping that behaviour.

I'm not sure.  iput_final() can take a lot of locks, both VFS and
heaven knows what within the individual filesystems.  Is it the case
that all allocations which occur under all of those locks is always
!__GFP_FS?  Hard to say...

> Threads that do have __GFP_FS in their gfp_mask can wait
> for the flusher in various ways.  Maybe the lock_page()
> method can be simplified by having the flusher thread
> unlock the page the moment it gets it, and then run the
> normal flusher code?

Well, _something_ has to pin the address_space.  A single locked page
will do.

> The pageout code (in shrink_page_list) already unlocks
> the page anyway before putting it back on the relevant
> LRU list.  It would be easy enough to skip that unlock
> and let the flusher thread take care of it.

Once that page is unlocked, we can't touch *mapping - its inode can be
concurrently reclaimed.  Although I guess the technique in
handle_write_error() can be reused.

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