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From: Theodore Ts'o <tytso@mit.edu>
To: Ming Lei <tom.leiming@gmail.com>
Cc: Jens Axboe <axboe@fb.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-block <linux-block@vger.kernel.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Mike Christie <mchristi@redhat.com>,
	Hannes Reinecke <hare@suse.com>,
	Keith Busch <keith.busch@intel.com>,
	Mike Snitzer <snitzer@redhat.com>,
	Johannes Thumshirn <jthumshirn@suse.de>,
	Bart Van Assche <bart.vanassche@sandisk.com>
Subject: Re: [PATCH 45/60] block: bio: introduce bio_for_each_segment_all_rd() and its write pair
Date: Tue, 1 Nov 2016 10:17:04 -0400	[thread overview]
Message-ID: <20161101141704.agxfiturhjhm2znn@thunk.org> (raw)
In-Reply-To: <CACVXFVPhS3FdzoRfqV8KFza_h0T48_bP-ix7y0_5exKw4iiXfg@mail.gmail.com>

On Tue, Nov 01, 2016 at 07:51:27AM +0800, Ming Lei wrote:
> Sorry for forgetting to mention one important point:
> 
> - after multipage bvec is introduced, the iterated bvec pointer
> still points to singlge page bvec, which is generated in-flight
> and is readonly actually. That is the motivation about the introduction
> of bio_for_each_segment_all_rd().
> 
> So maybe bio_for_each_page_all_ro() is better?
> 
> For _wt(), we still can keep it as bio_for_each_segment(), which also
> reflects that now the iterated bvec points to one whole segment if
> we name _rd as bio_for_each_page_all_ro().

I'm agnostic as to what the right names are --- my big concern is
there is an explosion of bio_for_each_page_* functions, and that there
isn't good documentation about (a) when to use each of these
functions, and (b) why.  I was goinig through the patch series, and it
was hard for me to figure out why, and I was looking through all of
the patches.  Once all of the patches are merged in, I am concerned
this is going to be massive trapdoor that will snare a large number of
unwitting developers.

As far as my preference, from an abstract perspective, if one version
(the read-write variant, I presume) is always safe, while one (the
read-only variant) is faster, if you can work under restricted
circumstances, naming the safe version so it is the "default", and
more dangerous one with the name that makes it a bit more obvious what
you have to do in order to use it safely, and then very clearly
document both in sources, and in the Documentation directory, what the
issues are and what you have to do in order to use the faster version.

Cheers,

					- Ted
					

  reply	other threads:[~2016-11-01 14:17 UTC|newest]

Thread overview: 107+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-10-29  8:07 [PATCH 00/60] block: support multipage bvec Ming Lei
2016-10-29  8:08 ` [PATCH 01/60] block: bio: introduce bio_init_with_vec_table() Ming Lei
2016-10-29 15:21   ` Christoph Hellwig
2016-10-29  8:08 ` [PATCH 02/60] block drivers: convert to bio_init_with_vec_table() Ming Lei
2016-10-29  8:08 ` [PATCH 03/60] block: drbd: remove impossible failure handling Ming Lei
2016-10-31 15:25   ` Christoph Hellwig
2016-10-29  8:08 ` [PATCH 04/60] block: floppy: use bio_add_page() Ming Lei
2016-10-31 15:26   ` Christoph Hellwig
2016-10-31 22:54     ` Ming Lei
2016-10-29  8:08 ` [PATCH 05/60] target: avoid to access .bi_vcnt directly Ming Lei
2016-10-31 15:26   ` Christoph Hellwig
2016-10-29  8:08 ` [PATCH 06/60] bcache: debug: avoid to access .bi_io_vec directly Ming Lei
2016-10-29  8:08 ` [PATCH 07/60] dm: crypt: use bio_add_page() Ming Lei
2016-10-29  8:08 ` [PATCH 08/60] dm: use bvec iterator helpers to implement .get_page and .next_page Ming Lei
2016-10-29  8:08 ` [PATCH 09/60] dm: dm.c: replace 'bio->bi_vcnt == 1' with !bio_multiple_segments Ming Lei
2016-10-31 15:29   ` Christoph Hellwig
2016-10-31 22:59     ` Ming Lei
2016-11-02  3:09     ` Kent Overstreet
2016-11-02  7:56       ` Ming Lei
2016-11-02 14:24         ` Mike Snitzer
2016-11-02 23:47           ` Ming Lei
2016-10-29  8:08 ` [PATCH 10/60] fs: logfs: convert to bio_add_page() in sync_request() Ming Lei
2016-10-29  8:08 ` [PATCH 11/60] fs: logfs: use bio_add_page() in __bdev_writeseg() Ming Lei
2016-10-31 15:29   ` Christoph Hellwig
2016-10-29  8:08 ` [PATCH 12/60] fs: logfs: use bio_add_page() in do_erase() Ming Lei
2016-10-31 15:29   ` Christoph Hellwig
2016-10-29  8:08 ` [PATCH 13/60] fs: logfs: remove unnecesary check Ming Lei
2016-10-31 15:29   ` Christoph Hellwig
2016-10-29  8:08 ` [PATCH 14/60] block: drbd: comment on direct access bvec table Ming Lei
2016-10-29  8:08 ` [PATCH 15/60] block: loop: comment on direct access to " Ming Lei
2016-10-31 15:31   ` Christoph Hellwig
2016-10-31 23:08     ` Ming Lei
2016-10-29  8:08 ` [PATCH 16/60] block: pktcdvd: " Ming Lei
2016-10-31 15:33   ` Christoph Hellwig
2016-10-31 23:08     ` Ming Lei
2016-10-29  8:08 ` [PATCH 17/60] kernel/power/swap.c: " Ming Lei
2016-10-29  8:08 ` [PATCH 18/60] mm: page_io.c: " Ming Lei
2016-10-29  8:08 ` [PATCH 19/60] fs/buffer: " Ming Lei
2016-10-31 15:35   ` Christoph Hellwig
2016-10-31 23:12     ` Ming Lei
2016-10-29  8:08 ` [PATCH 20/60] f2fs: f2fs_read_end_io: " Ming Lei
2016-10-29  8:08 ` [PATCH 21/60] bcache: " Ming Lei
2016-10-29  8:08 ` [PATCH 22/60] block: comment on bio_alloc_pages() Ming Lei
2016-10-29  8:08 ` [PATCH 23/60] block: introduce flag QUEUE_FLAG_NO_MP Ming Lei
2016-10-29 15:29   ` Christoph Hellwig
2016-10-29 22:20     ` Ming Lei
2016-10-29  8:08 ` [PATCH 24/60] md: set NO_MP for request queue of md Ming Lei
2016-10-29  8:08 ` [PATCH 25/60] block: pktcdvd: set NO_MP for pktcdvd request queue Ming Lei
2016-10-29  8:08 ` [PATCH 26/60] btrfs: set NO_MP for request queues behind BTRFS Ming Lei
2016-10-31 15:36   ` Christoph Hellwig
2016-10-31 17:58     ` Chris Mason
2016-10-31 18:00       ` Christoph Hellwig
2016-10-29  8:08 ` [PATCH 27/60] block: introduce BIO_SP_MAX_SECTORS Ming Lei
2016-10-29  8:08 ` [PATCH 28/60] block: introduce QUEUE_FLAG_SPLIT_MP Ming Lei
2016-10-31 15:39   ` Christoph Hellwig
2016-10-31 23:56     ` Ming Lei
2016-11-02  3:08     ` Kent Overstreet
2016-11-03 10:38       ` Ming Lei
2016-11-03 11:20         ` Kent Overstreet
2016-11-03 11:26           ` Ming Lei
2016-11-03 11:30             ` Kent Overstreet
2016-10-29  8:08 ` [PATCH 29/60] dm: limit the max bio size as BIO_SP_MAX_SECTORS << SECTOR_SHIFT Ming Lei
2016-10-29  8:08 ` [PATCH 30/60] bcache: set flag of QUEUE_FLAG_SPLIT_MP Ming Lei
2016-10-29  8:08 ` [PATCH 31/60] block: introduce multipage/single page bvec helpers Ming Lei
2016-10-29  8:08 ` [PATCH 32/60] block: implement sp version of bvec iterator helpers Ming Lei
2016-10-29 11:06   ` kbuild test robot
2016-12-17 11:38     ` Ming Lei
2016-10-29  8:08 ` [PATCH 33/60] block: introduce bio_for_each_segment_mp() Ming Lei
2016-10-29  8:08 ` [PATCH 34/60] block: introduce bio_clone_sp() Ming Lei
2016-10-29  8:08 ` [PATCH 35/60] bvec_iter: introduce BVEC_ITER_ALL_INIT Ming Lei
2016-10-29  8:08 ` [PATCH 36/60] block: bounce: avoid direct access to bvec from bio->bi_io_vec Ming Lei
2016-10-29  8:08 ` [PATCH 37/60] block: bounce: don't access bio->bi_io_vec in copy_to_high_bio_irq Ming Lei
2016-10-29  8:08 ` [PATCH 38/60] block: bounce: convert multipage bvecs into singlepage Ming Lei
2016-10-29  8:08 ` [PATCH 39/60] bcache: debug: switch to bio_clone_sp() Ming Lei
2016-10-29  8:08 ` [PATCH 40/60] blk-merge: compute bio->bi_seg_front_size efficiently Ming Lei
2016-10-29  8:08 ` [PATCH 41/60] block: blk-merge: try to make front segments in full size Ming Lei
2016-10-29  8:08 ` [PATCH 42/60] block: use bio_for_each_segment_mp() to compute segments count Ming Lei
2016-10-29  8:08 ` [PATCH 43/60] block: use bio_for_each_segment_mp() to map sg Ming Lei
2016-10-29  8:08 ` [PATCH 44/60] block: introduce bvec_for_each_sp_bvec() Ming Lei
2016-10-29  8:08 ` [PATCH 45/60] block: bio: introduce bio_for_each_segment_all_rd() and its write pair Ming Lei
2016-10-31 13:59   ` Theodore Ts'o
2016-10-31 15:11     ` Christoph Hellwig
2016-10-31 22:50       ` Ming Lei
2016-11-02  3:01       ` Kent Overstreet
2016-10-31 22:46     ` Ming Lei
2016-10-31 23:51       ` Ming Lei
2016-11-01 14:17         ` Theodore Ts'o [this message]
2016-11-02  1:58           ` Ming Lei
2016-10-29  8:08 ` [PATCH 46/60] block: deal with dirtying pages for multipage bvec Ming Lei
2016-10-31 15:40   ` Christoph Hellwig
2016-11-01  0:19     ` Ming Lei
2016-10-29  8:08 ` [PATCH 47/60] block: convert to bio_for_each_segment_all_rd() Ming Lei
2016-10-29  8:08 ` [PATCH 48/60] fs/mpage: " Ming Lei
2016-10-29  8:08 ` [PATCH 49/60] fs/direct-io: " Ming Lei
2016-10-29  8:08 ` [PATCH 50/60] ext4: " Ming Lei
2016-10-29  8:08 ` [PATCH 51/60] xfs: " Ming Lei
2016-10-29  8:08 ` [PATCH 52/60] logfs: " Ming Lei
2016-10-29  8:08 ` [PATCH 53/60] gfs2: " Ming Lei
2016-10-29  8:08 ` [PATCH 54/60] f2fs: " Ming Lei
2016-10-29  8:08 ` [PATCH 55/60] exofs: " Ming Lei
2016-10-29  8:08 ` [PATCH 56/60] fs: crypto: " Ming Lei
2016-10-29  8:08 ` [PATCH 57/60] bcache: " Ming Lei
2016-10-29  8:08 ` [PATCH 58/60] dm-crypt: " Ming Lei
2016-10-29  8:08 ` [PATCH 59/60] fs/buffer.c: use bvec iterator to truncate the bio Ming Lei
2016-10-29  8:08 ` [PATCH 60/60] block: enable multipage bvecs Ming Lei
2016-10-31 15:25 ` [PATCH 00/60] block: support multipage bvec Christoph Hellwig
2016-10-31 22:52   ` Ming Lei

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