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List:       linux-bcache
Subject:    Re: [PATCH 1/6] block: add a bio_reuse helper
From:       Kent Overstreet <kent.overstreet () gmail ! com>
Date:       2018-06-13 14:49:52
Message-ID: 20180613144952.GA17292 () kmo-pixel
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On Wed, Jun 13, 2018 at 03:59:15PM +0200, Christoph Hellwig wrote:
> On Wed, Jun 13, 2018 at 04:54:41AM -0400, Kent Overstreet wrote:
> > bi_size is not immutable though, it will usually be modified by drivers when you
> > submit a bio.
> > 
> > I see what you're trying to do, but your approach is busted given the way the
> > block layer works today. You'd have to save bio->bi_iter before submitting the
> > bio and restore it afterwards for it to work.
> 
> For bi_size, agreed this needs fixing.  bi_sector is always restored
> already by the callers, and the remaining fields are zeroed by bio_reset,
> which does the right thing.

This still shouldn't be a new helper. If the caller is restoring bi_sector they
can restore bi_size as well, and restoring only part of bi_iter should not be
encouraged as it's not safe in general - it is a quite reasonable thing to want
to restore a bio to the state it was pre submission (e.g. for bouncing) and what
you're doing is definitely _not_ safe in general.
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