Re: [dm-devel] why does __split_and_process_bio use bio_clone_bioset?

[NeilBrown <neilb@suse.com>]

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On Thu, Jun 14 2018, Mike Snitzer wrote:

> On Thu, Jun 14 2018 at  2:12P -0400,
> Mike Snitzer <snitzer@redhat.com> wrote:
>
>> On Thu, Jun 14 2018 at  4:19am -0400,
>> Christoph Hellwig <hch@infradead.org> wrote:
>> 
>> > Hi Neil,
>> > 
>> > In commit 18a25da8 ("dm: ensure bio submission follows a depth-first
>> > tree walk") you've added a call to bio_clone_bioset to
>> > __split_and_process_bio.  Unlike all other bio splitting code this
>> > actually allocates a new bio_vec array instead of just splitting the bio
>> > and the iterator.  I can't actually find a good reason for that either
>> > in a cursory review of the code, the commit or the comments.
>> >
>> > Do you remember why this can't just use bio_clone_fast?

Good question.  I don't remember having a good reason to choose it, and
if there was one I suspect I would have mentioned it in the commit
message.
So it was most likely an oversight.
Looking at the code now, I can see no justification for not using
bio_clone_fast() or similar.

Thanks for looking into this - I guess the next step is to get rid of
bio_clone_bioset() completely.  Nice.


>> 
>> Your question caused me to revisit this code and it is suspect for a
>> couple reasons:
>> 
>> 1) I'm also not seeing why we need bio_clone_bioset()
>
> The patch below seems to work fine (given quick testing).. It also has a
> side-effect of not breaking integrity support (which commit 18a25da8
> appears to do because it isn't accounting for any of the integrity stuff
> bio_split, or dm.c:clone_bio, does).
>
> FYI, my other concerns in my my previous reply were unfounded and due to
> misreading the existing code.
>
> Neil, please still feel free to have a look at this to see if you can
> recall why you used bio_clone_bioset().
>
> If in the end you agree that the following patch is fine please let me
> know and I'll get a proper fix staged.

I agree with the patch.
 Reviewed-by: NeilBrown <neilb@suse.com>

Thanks!

NeilBrown

>
> Thanks,
> Mike
>
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 20a8d63..dfb4783 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1582,10 +1582,9 @@ static blk_qc_t __split_and_process_bio(struct mapped_device *md,
>  				 * the usage of io->orig_bio in dm_remap_zone_report()
>  				 * won't be affected by this reassignment.
>  				 */
> -				struct bio *b = bio_clone_bioset(bio, GFP_NOIO,
> -								 &md->queue->bio_split);
> +				struct bio *b = bio_split(bio, bio_sectors(bio) - ci.sector_count,
> +							  GFP_NOIO, &md->queue->bio_split);
>  				ci.io->orig_bio = b;
> -				bio_advance(bio, (bio_sectors(bio) - ci.sector_count) << 9);
>  				bio_chain(b, bio);
>  				ret = generic_make_request(bio);
>  				break;
>
> --
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