linux-kernel - Re: [PATCH v5 10/11] block: remove bio_get_nr

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Date:	Tue, 07 Jul 2015 10:04:01 +0100
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	Ming Lin <mlin@...nel.org>
CC:	lkml <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Dongsu Park <dpark@...teo.net>,
	Mike Snitzer <snitzer@...hat.com>,
	Ming Lin <ming.l@....samsung.com>
Subject: Re: [PATCH v5 10/11] block: remove bio_get_nr_vecs()

Hi,

On 06/07/15 18:21, Ming Lin wrote:
> On Mon, Jul 6, 2015 at 3:58 AM, Steven Whitehouse <swhiteho@...hat.com> wrote:
>> Hi,
>>
>>
>> On 06/07/15 08:44, Ming Lin wrote:
>>> From: Kent Overstreet <kent.overstreet@...il.com>
>>>
>>> We can always fill up the bio now, no need to estimate the possible
>>> size based on queue parameters.
>> [snip]
>>> diff --git a/fs/gfs2/lops.c b/fs/gfs2/lops.c
>>> index 2c1ae86..64d3116 100644
>>> --- a/fs/gfs2/lops.c
>>> +++ b/fs/gfs2/lops.c
>>> @@ -261,18 +261,11 @@ void gfs2_log_flush_bio(struct gfs2_sbd *sdp, int rw)
>>>    static struct bio *gfs2_log_alloc_bio(struct gfs2_sbd *sdp, u64 blkno)
>>>    {
>>>          struct super_block *sb = sdp->sd_vfs;
>>> -       unsigned nrvecs = bio_get_nr_vecs(sb->s_bdev);
>>>          struct bio *bio;
>>>          BUG_ON(sdp->sd_log_bio);
>>>    -     while (1) {
>>> -               bio = bio_alloc(GFP_NOIO, nrvecs);
>>> -               if (likely(bio))
>>> -                       break;
>>> -               nrvecs = max(nrvecs/2, 1U);
>>> -       }
>>> -
>>> +       bio = bio_alloc(GFP_NOIO, BIO_MAX_PAGES);
>>>          bio->bi_iter.bi_sector = blkno * (sb->s_blocksize >> 9);
>>>          bio->bi_bdev = sb->s_bdev;
>>>          bio->bi_end_io = gfs2_end_log_write;
>>
>> When I wrote this loop I understood that bio_alloc(GFP_NOIO, x) was only guaranteed to be successful in the case x = 1, and that for x > 1, it might return NULL. Does this now mean that for x = BIO_MAX_PAGES it will be guaranteed to never return NULL in this case?
> Yes, it's guaranteed.
>
> https://lkml.org/lkml/2013/11/26/47
>
> commit df2cb6daa4cbc

Ok. Feel free to add:
Acked-by: Steven Whitehouse <swhiteho@...hat.com>

Steve.

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