linux-kernel - Re: [PATCH v5 11/12] block: Add bio_clone

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Message-ID: <20120809065251.GD2845@dhcp-172-17-108-109.mtv.corp.google.com>
Date:	Wed, 8 Aug 2012 23:52:51 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, axboe@...nel.dk, agk@...hat.com,
	neilb@...e.de, drbd-dev@...ts.linbit.com, vgoyal@...hat.com,
	mpatocka@...hat.com, sage@...dream.net, yehuda@...newdream.net
Subject: Re: [PATCH v5 11/12] block: Add bio_clone_bioset()

On Wed, Aug 08, 2012 at 07:56:10PM -0700, Kent Overstreet wrote:
> > So, bio_clone() loses its function comment.  Also, does it even make
> > sense to call bio_clone() from fs_bio_set?
> 
> I'll re add the function comment if you want, just for a single line
> wrapper I don't know if it's worth the cost - comments get out of date,
> and they're more stuff to wade through.

People actually look at docbook generated docs.  I don't know why but
they do.  It's a utility function at block layer.  Please just add the
comment.

> > Let's say it's so, then
> > what's the difference from using _kmalloc variant?
> 
> bio_kmalloc() fails if nr_iovecs > 1024, bio_alloc_bioset() fails if
> nr_iovecs > 256
> 
> and bio_alloc_bioset() is mempool backed, bio_kmalloc() is not.
> 
> AFAICT that's it.

So, the thing is being mempool backed doesn't mean anything if
multiple layers use the pool.  I *suspect* fs_bio_set is supposed to
be used by fs layer - ie. where bios originate.  The reason why I
wondered about bio_clone() is that bio_clone() is almost always used
from stacking drivers and stacking driver tapping into fs reserve is
buggy.  So, I'm wondering whether cloning from fs_bio_set should be
supported at all.

Thanks.

-- 
tejun
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