linux-kernel - Re: [PATCH 11/11] block: don't check for BIO_MAX_PAGES in blk_bio_segment

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Date:   Mon, 24 Apr 2017 13:16:21 +1000
From:   NeilBrown <neilb@...e.com>
To:     Ming Lei <tom.leiming@...il.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/11] block: don't check for BIO_MAX_PAGES in blk_bio_segment_split()

On Fri, Apr 21 2017, Ming Lei wrote:

> On Fri, Apr 21, 2017 at 7:34 PM, Christoph Hellwig <hch@...radead.org> wrote:
>> On Thu, Apr 20, 2017 at 03:38:50PM +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.
>>
>> Hmm.  From Jens tree:
>>
>> drivers/lightnvm/pblk-read.c:           int_bio = bio_clone_bioset(bio, GFP_KERNEL, fs_bio_set);
>> drivers/md/raid1.c:                             mbio = bio_clone_bioset_partial(bio, GFP_NOIO,
>> drivers/md/raid1.c:                             mbio = bio_clone_bioset_partial(bio, GFP_NOIO,
>
> Btrfs use bio_clone_bioset() too:
>
>      fs/btrfs/extent_io.c:2703:      new = bio_clone_bioset(bio,
> gfp_mask, btrfs_bioset);
>

btrfs is a filesystem, not a driver.  So it doesn't count.
The bios it uses were not yet processed by blk_bio_segment_split(),
so changes there cannot be relevant to btrfs.

Thanks,
NeilBrown

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