linux-kernel - Re: [PATCH v4 06/11] md/raid5: get rid of bio_fits

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Message-ID: <CAF1ivSZee_aV4XOtgc9YOR4YG4Lsd0tJkE-F+i9jg-5Ep7_Afg@mail.gmail.com>
Date:	Tue, 26 May 2015 16:42:35 -0700
From:	Ming Lin <mlin@...nel.org>
To:	NeilBrown <neilb@...e.de>
Cc:	Ming Lin <mlin@...nel.org>, Christoph Hellwig <hch@....de>,
	lkml <linux-kernel@...r.kernel.org>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Jens Axboe <axboe@...nel.dk>, Dongsu Park <dpark@...teo.net>,
	linux-raid@...r.kernel.org
Subject: Re: [PATCH v4 06/11] md/raid5: get rid of bio_fits_rdev()

On Tue, May 26, 2015 at 4:03 PM, NeilBrown <neilb@...e.de> wrote:
> On Tue, 26 May 2015 15:32:38 -0700 Ming Lin <mlin@...nel.org> wrote:
>
>> On Tue, May 26, 2015 at 7:33 AM, Ming Lin <mlin@...nel.org> wrote:
>> > On Mon, May 25, 2015 at 7:17 AM, Christoph Hellwig <hch@....de> wrote:
>> >> On Mon, May 25, 2015 at 05:54:14PM +1000, NeilBrown wrote:
>> >>> Did I write that?  I guess I did :-(
>> >>> I meant *after*.   Don't get rid of bio_fits_rdev until split_bio is in
>> >>> chunk_aligned_read().
>> >>
>> >> I suspect the whole series could use some reordering.
>> >
>> > Nice reordering.
>> > I'll do this.
>>
>> Here is the reordering.
>> https://git.kernel.org/cgit/linux/kernel/git/mlin/linux.git/log/?h=block-generic-req
>>
>> I'll post it if you are OK.
>>
>> [PATCH 01/15] block: add blk_queue_split()
>> [PATCH 02/15] md: remove ->merge_bvec_fn
>> [PATCH 03/15] dm: remov merge functions
>> [PATCH 04/15] drbd: remove ->merge_bvec_fn
>> [PATCH 05/15] pktcdvd: remove ->merge_bvec_fn
>> [PATCH 06/15] rbd: remove ->merge_bvec_fn
>> [PATCH 07/15] bcache: remove driver private bio splitting code
>> [PATCH 08/15] btrfs: remove bio splitting and merge_bvec_fn() calls
>> [PATCH 09/15] block: call blk_queue_split() in make_request functions
>> [PATCH 10/15] block: kill ->merge_bvec_fn and simplify bio_add_page
>> [PATCH 11/15] block: remove split code in blkdev_issue_discard
>> [PATCH 12/15] md/raid5: get rid of bio_fits_rdev()
>> [PATCH 13/15] block: remove bio_get_nr_vecs()
>> [PATCH 14/15] fs: use helper bio_add_page() instead of open coding on
>> [PATCH 15/15] Documentation: update notes in biovecs about
>
> The changes to dm.c and dm.h should be in the "dm:" patch, not "md:".

Will move it.

>
> But I don't think the sequence is right.
>
> You cannot remove ->merge_bvec_fn for *any* stacked device until *all* devices
> make use of blk_queue_split() (or otherwise handle arbitrarily large bios).
>
> I think it would be easiest to:
>  - add blk_queue_split() and call it from common code before ->make_request_fn
>    is called.  The ensure all devices can accept arbitrarily large bios.

For "common code", do you mean "generic_make_request()"

diff --git a/block/blk-core.c b/block/blk-core.c
index fbbb337..bb6455b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1942,6 +1942,7 @@ void generic_make_request(struct bio *bio)
        do {
                struct request_queue *q = bdev_get_queue(bio->bi_bdev);

+               blk_queue_split(q, &bio, q->bio_split);
                q->make_request_fn(q, bio);

                bio = bio_list_pop(current->bio_list);

>  - driver-by-driver remove merge_bvec_fn and make sure the the driver can cope
>    with arbitrary bios themselve, calling blk_queue_split in the make_request
>    function only if needed
>  - finally remove the call to blk_queue_split from the common code.
>
> Does that make sense to others?
>
> Thanks,
> NeilBrown
>
>>
>> >
>> > Thanks.
>> >
>> >>
>> >> patch 1:
>> >>
>> >>  add ->bio_split and blk_queue_split
>> >>
>> >> patch 2..n:
>> >>
>> >>  one for each non-trivial driver that implements ->merge_bvec_fn to
>> >>  remove it and instead split bios in ->make_request.  The md patch
>> >>  to do the right thing in chunk_aligned_read goes into the general
>> >>  md patch here.  The bcache patch also goes into this series.
>> >>
>> >> patch n+1:
>> >>
>> >>  - add blk_queue_split calls for remaining trivial drivers
>> >>
>> >> patch n+2:
>> >>
>> >>  - remove ->merge_bvec_fn and checking of max_sectors a for all
>> >>    drivers, simplify bio_add_page
>> >>
>> >> patch n+2:
>> >>
>> >>  - remove splitting in blkdev_issue_discard
>> >>
>> >> patch n+3
>> >>
>> >>  - remove bio_fits_rdev
>> >>
>> >> patch n+4
>> >>
>> >>  - remove bio_get_nr_vecs
>> >>
>> >> patch n+4
>> >>
>> >>  - use bio_add_page
>> >>
>> >> patch n+5
>> >>
>> >>  - update documentation
>
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