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

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Date:	Mon, 6 Jul 2015 10:21:52 -0700
From:	Ming Lin <mlin@...nel.org>
To:	Steven Whitehouse <swhiteho@...hat.com>
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()

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
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