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

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Message-ID: <20150527090309.2cddf33b@notabene.brown>
Date:	Wed, 27 May 2015 09:03:09 +1000
From:	NeilBrown <neilb@...e.de>
To:	Ming Lin <mlin@...nel.org>
Cc:	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, 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:".

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.
 - 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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