linux-kernel - Re: [dm-devel] [PATCH v10 1/8] block: Generalized bio pool freeing

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Message-ID: <20120917205139.GB14492@google.com>
Date:	Mon, 17 Sep 2012 13:51:39 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, Jens Axboe <axboe@...nel.dk>,
	Lars Ellenberg <lars.ellenberg@...bit.com>,
	Hannes Reinecke <hare@...e.de>
Subject: Re: [dm-devel] [PATCH v10 1/8] block: Generalized bio pool freeing

On Fri, Sep 14, 2012 at 07:28:28PM +0100, Alasdair G Kergon wrote:
> On Thu, Sep 06, 2012 at 03:34:55PM -0700, Kent Overstreet wrote:
> > With the old code, when you allocate a bio from a bio pool you have to
> > implement your own destructor that knows how to find the bio pool the
> > bio was originally allocated from.
> > 
> > This adds a new field to struct bio (bi_pool) and changes
> > bio_alloc_bioset() to use it. This makes various bio destructors
> > unnecessary, so they're then deleted.
> > 
> > v6: Explain the temporary if statement in bio_put
>  
> This patch also silently reverts 
> commit 4d7b38b7d944a79da3793b6c92d38682f3905ac9
> "dm: clear bi_end_io on remapping failure"
> 
> Why?
> 
> If it's intentional, please explain it in your patch header and
> copy Hannes to reconsider the matter.

Never noticed that was introduced in its own patch until you pointed it
out.

That isn't a very good patch - it says it's clearing bi_end_io as a
precaution, but as a precaution to what?

As far as I can tell, it was never necessary. The bio is about to be
freed - there shouldn't be any other references on it (__bio_map() is
called on freshly allocated bios, and bio_get() is never called in dm.c)
Nothing else should've been looking at bi_end_io, certainly the
destructor didn't.

Now that there's no destructor, it makes even less sense to have it -
after that bio_put() that bio isn't being touched by dm code anymore.
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