linux-kernel - Re: [PATCH 13/13] block: don't check for BIO_MAX_PAGES in blk_bio_segment

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Message-ID: <87tw52q37k.fsf@notabene.neil.brown.name>
Date:   Wed, 03 May 2017 08:54:55 +1000
From:   NeilBrown <neilb@...e.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Ming Lei <tom.leiming@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/13] block: don't check for BIO_MAX_PAGES in blk_bio_segment_split()

On Tue, May 02 2017, Ming Lei wrote:

> On Tue, May 02, 2017 at 01:42:26PM +1000, NeilBrown wrote:
>> blk_bio_segment_split() makes sure bios have no more than
>> BIO_MAX_PAGES entries in the bi_io_vec.
>> This was done because bio_clone_bioset() (when given a
>> mempool bioset) could not handle larger io_vecs.
>> 
>> No driver uses bio_clone_bioset() any more, they all
>> use bio_clone_fast() if anything, and bio_clone_fast()
>> doesn't clone the bi_io_vec.
>
> Maybe in future, some drivers still may try to use 
> bio_clone_bioset() again, I suggest to add some comments
> on bio_clone_bioset() to make this usage explicitly. Also
> better to trigger a warning if a big src bio is passed to
> bio_clone_bioset().

There are now just two users for bio_clone_bioset(): bounce.c and btrfs.

Christoph wants to get rid of bounce.c, which would leave one.

I'd have to drill into the btrfs code to be sure, but it might be that
btrfs only needs bio_clone_fast().  That would leave zero users.
Then we wouldn't need a warning at all.

So I agree that we need to guard against future incorrect usage.  I'm not
yet sure what the best approach is.

Thanks,
NeilBrown

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