Date:	Wed, 14 Apr 2010 17:28:30 +1000
From:	Dave Chinner <david@...morbit.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Chris Mason <chris.mason@...cle.com>, Mel Gorman <mel@....ul.ie>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm: disallow direct reclaim page writeback

On Wed, Apr 14, 2010 at 03:52:44PM +0900, KOSAKI Motohiro wrote:
> > On Tue, Apr 13, 2010 at 04:20:21PM -0400, Chris Mason wrote:
> > > On Tue, Apr 13, 2010 at 08:34:29PM +0100, Mel Gorman wrote:
> > > > > Basically, there is not enough stack space available to allow direct
> > > > > reclaim to enter ->writepage _anywhere_ according to the stack usage
> > > > > profiles we are seeing here....
> > > > > 
> > > > 
> > > > I'm not denying the evidence but how has it been gotten away with for years
> > > > then? Prevention of writeback isn't the answer without figuring out how
> > > > direct reclaimers can queue pages for IO and in the case of lumpy reclaim
> > > > doing sync IO, then waiting on those pages.
> > > 
> > > So, I've been reading along, nodding my head to Dave's side of things
> > > because seeks are evil and direct reclaim makes seeks.  I'd really loev
> > > for direct reclaim to somehow trigger writepages on large chunks instead
> > > of doing page by page spatters of IO to the drive.
> 
> I agree that "seeks are evil and direct reclaim makes seeks". Actually,
> making 4k io is not must for pageout. So, probably we can improve it.
> 
> 
> > Perhaps drop the lock on the page if it is held and call one of the
> > helpers that filesystems use to do this, like:
> > 
> > 	filemap_write_and_wait(page->mapping);
> 
> Sorry, I'm lost what you talk about. Why do we need per-file
> waiting? If file is 1GB file, do we need to wait 1GB writeout?

So use filemap_fdatawrite(page->mapping), or if it's better only
to start IO on a segment of the file, use
filemap_fdatawrite_range(page->mapping, start, end)....

> > > But, somewhere along the line I overlooked the part of Dave's stack trace
> > > that said:
> > > 
> > > 43)     1568     912   do_select+0x3d6/0x700
> > > 
> > > Huh, 912 bytes...for select, really?  From poll.h:
> > 
> > Sure, it's bad, but we focussing on the specific case misses the
> > point that even code that is using minimal stack can enter direct
> > reclaim after consuming 1.5k of stack. e.g.:
> 
> checkstack.pl says do_select() and __generic_file_splice_read() are one
> of worstest stack consumer. both sould be fixed.

the deepest call chain in queue_work() needs 700 bytes of stack
to complete, wait_for_completion() requires almost 2k of stack space
at it's deepest, the scheduler has some heavy stack users, etc,
and these are all functions that appear at the top of the stack.

> also, checkstack.pl says such stack eater aren't so much.

Yeah, but when we have ia callchain 70 or more functions deep,
even 100 bytes of stack is a lot....

> > > So, select is intentionally trying to use that much stack.  It should be using
> > > GFP_NOFS if it really wants to suck down that much stack...
> > 
> > The code that did the allocation is called from multiple different
> > contexts - how is it supposed to know that in some of those contexts
> > it is supposed to treat memory allocation differently?
> > 
> > This is my point - if you introduce a new semantic to memory allocation
> > that is "use GFP_NOFS when you are using too much stack" and too much
> > stack is more than 15% of the stack, then pretty much every code path
> > will need to set that flag...
> 
> Nodding my head to Dave's side. changing caller argument seems not good
> solution. I mean
>  - do_select() should use GFP_KERNEL instead stack (as revert 70674f95c0)
>  - reclaim and xfs (and other something else) need to diet.

The list I'm seeing so far includes:
	- scheduler
	- completion interfaces
	- radix tree
	- memory allocation, memory reclaim
	- anything that implements ->writepage
	- select
	- splice read

> Also, I believe stack eater function should be created waring. patch attached.

Good start, but 512 bytes will only catch select and splice read,
and there are 300-400 byte functions in the above list that sit near
the top of the stack....

> > We need at least _700_ bytes of stack free just to call queue_work(),
> > and that now happens deep in the guts of the driver subsystem below XFS.
> > This trace shows 1.8k of stack usage on a simple, single sata disk
> > storage subsystem, so my estimate of 2k of stack for the storage system
> > below XFS is too small - a worst case of 2.5-3k of stack space is probably
> > closer to the mark.
> 
> your explanation is very interesting. I have a (probably dumb) question.
> Why nobody faced stack overflow issue in past? now I think every users
> easily get stack overflow if your explanation is correct.

It's always a problem, but the focus on minimising stack usage has
gone away since i386 has mostly disappeared from server rooms.

XFS has always been the thing that triggered stack usage problems
first - the first reports of problems on x86_64 with 8k stacks in low
memory situations have only just come in, and this is the first time
in a couple of years I've paid close attention to stack usage
outside XFS. What I'm seeing is not pretty....

> > This is the sort of thing I'm pointing at when I say that stack
> > usage outside XFS has grown significantly significantly over the
> > past couple of years. Given XFS has remained pretty much the same or
> > even reduced slightly over the same time period, blaming XFS or
> > saying "callers should use GFP_NOFS" seems like a cop-out to me.
> > Regardless of the IO pattern performance issues, writeback via
> > direct reclaim just uses too much stack to be safe these days...
> 
> Yeah, My answer is simple, All stack eater should be fixed.
> but XFS seems not innocence too. 3.5K is enough big although
> xfs have use such amount since very ago.

XFS used to use much more than that - significant effort has been
put into reduce the stack footprint over many years. There's not
much left to trim without rewriting half the filesystem...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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