linux-kernel - Re: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by stacking drivers

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Message-ID: <20120904182633.GB3638@dhcp-172-17-108-109.mtv.corp.google.com>
Date:	Tue, 4 Sep 2012 11:26:33 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Dave Chinner <david@...morbit.com>,
	Kent Overstreet <koverstreet@...gle.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, bharrosh@...asas.com,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by
 stacking drivers

Hello,

On Tue, Sep 04, 2012 at 09:54:23AM -0400, Vivek Goyal wrote:
> > Given that we are working around stack depth issues in the
> > filesystems already in several places, and now it seems like there's
> > a reason to work around it in the block layers as well, shouldn't we
> > simply increase the default stack size rather than introduce
> > complexity and performance regressions to try and work around not
> > having enough stack?
> 
> Dave,
> 
> In this particular instance, we really don't have any bug reports of
> stack overflowing. Just discussing what will happen if we make 
> generic_make_request() recursive again.

I think there was one and that's why we added the bio_list thing.

> > I mean, we can deal with it like the ia32 4k stack issue was dealt
> > with (i.e. ignore those stupid XFS people, that's an XFS bug), or
> > we can face the reality that storage stacks have become so complex
> > that 8k is no longer a big enough stack for a modern system....
> 
> So first question will be, what's the right stack size? If we make
> generic_make_request() recursive, then at some storage stack depth we will
> overflow stack anyway (if we have created too deep a stack). Hence
> keeping current logic kind of makes sense as in theory we can support
> arbitrary depth of storage stack.

But, yeah, this can't be solved by enlarging the stack size.  The
upper limit is unbound.

Thanks.

-- 
tejun
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