Message-ID: <Pine.LNX.4.64.1205181823540.26504@file.rdu.redhat.com>
Date:	Fri, 18 May 2012 18:48:37 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	device-mapper development <dm-devel@...hat.com>
cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tj@...nel.org, axboe@...nel.dk,
	Kent Overstreet <koverstreet@...gle.com>, agk@...hat.com
Subject: Re: [dm-devel] [PATCH 12/13] Make generic_make_request handle
 arbitrarily large bios

Hi

This patch looks very good, this approach will hopefully fix the long 
standing bug - when you add usb disk to dm-mirro or md-raid-1, requests 
would fail because the usb disk has different limits.

bio_segments returns the number of pages (not segments) in the bio. And 
this is incorrectly compared to queue_max_segments. It should compare 
bio_phys_segments to queue_max_segments.


If you have this split infrastructure, it would be good if you allow to 
use it by other drivers - I'd suggest that you change q->make_request_fn 
to return 0 on success (bio submitted) and a number of sectors on failure 
(this means that the driver only accepts the specified number of sector 
and --- generic_make_request needs to split the bio and submit a smaller 
bio with the specified number of sectors). If you do it this way, md and 
dm can be simplified - they will no longer need to do their own request 
splitting and they could just return a number of sectors they accept.

What I mean:
generic_make_request calls q->make_request_fn(q, bio). If the return value 
is zero, continue with the next bio. If the return value is nonzero, split 
the current bio so that the first part of the bio has the number of 
sectors specified in the return value. Submit the partial bio (normally it 
should be accepted, but in the extreme case when someone changed dm 
mapping underneath, we would need to split it again) and the remainder.

The check bio_sectors(bio) <= bio_max_sectors(bio) can then be moved to 
blk_queue_bio.

This way, we could remove bio splitting from dm and md and simplify them.

Mikulas



On Thu, 17 May 2012, koverstreet@...gle.com wrote:

> From: Kent Overstreet <koverstreet@...gle.com>
> 
> The way the block layer is currently written, it goes to great lengths
> to avoid having to split bios; upper layer code (such as bio_add_page())
> checks what the underlying device can handle and tries to always create
> bios that don't need to be split.
> 
> But this approach becomes unwieldy and eventually breaks down with
> stacked devices and devices with dynamic limits, and it adds a lot of
> complexity. If the block layer could split bios as needed, we could
> eliminate a lot of complexity elsewhere - particularly in stacked
> drivers. Code that creates bios can then create whatever size bios are
> convenient, and more importantly stacked drivers don't have to deal with
> both their own bio size limitations and the limitations of the
> (potentially multiple) devices underneath them.
> 
> Signed-off-by: Kent Overstreet <koverstreet@...gle.com>
> ---
>  block/blk-core.c       |  111 ++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/bio.c               |   41 ++++++++++++++++++
>  include/linux/bio.h    |    7 +++
>  include/linux/blkdev.h |    3 +
>  4 files changed, 162 insertions(+), 0 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 91617eb..1d7a482 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -29,6 +29,7 @@
>  #include <linux/fault-inject.h>
>  #include <linux/list_sort.h>
>  #include <linux/delay.h>
> +#include <linux/closure.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/block.h>
> @@ -52,6 +53,12 @@ static struct kmem_cache *request_cachep;
>  struct kmem_cache *blk_requestq_cachep;
>  
>  /*
> + * For bio_split_hook
> + */
> +static struct kmem_cache *bio_split_cache;
> +static struct workqueue_struct *bio_split_wq;
> +
> +/*
>   * Controlling structure to kblockd
>   */
>  static struct workqueue_struct *kblockd_workqueue;
> @@ -487,6 +494,14 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
>  	if (q->id < 0)
>  		goto fail_q;
>  
> +	q->bio_split_hook = mempool_create_slab_pool(4, bio_split_cache);
> +	if (!q->bio_split_hook)
> +		goto fail_split_hook;
> +
> +	q->bio_split = bioset_create(4, 0);
> +	if (!q->bio_split)
> +		goto fail_split;
> +
>  	q->backing_dev_info.ra_pages =
>  			(VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
>  	q->backing_dev_info.state = 0;
> @@ -526,6 +541,10 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
>  
>  fail_id:
>  	ida_simple_remove(&blk_queue_ida, q->id);
> +fail_split:
> +	bioset_free(q->bio_split);
> +fail_split_hook:
> +	mempool_destroy(q->bio_split_hook);
>  fail_q:
>  	kmem_cache_free(blk_requestq_cachep, q);
>  	return NULL;
> @@ -1493,6 +1512,83 @@ static inline bool should_fail_request(struct hd_struct *part,
>  
>  #endif /* CONFIG_FAIL_MAKE_REQUEST */
>  
> +struct bio_split_hook {
> +	struct closure		cl;
> +	struct request_queue	*q;
> +	struct bio		*bio;
> +	bio_end_io_t		*bi_end_io;
> +	void			*bi_private;
> +};
> +
> +static void bio_submit_split_done(struct closure *cl)
> +{
> +	struct bio_split_hook *s = container_of(cl, struct bio_split_hook, cl);
> +
> +	s->bio->bi_end_io	= s->bi_end_io;
> +	s->bio->bi_private	= s->bi_private;
> +	bio_endio(s->bio, 0);
> +
> +	closure_debug_destroy(&s->cl);
> +	mempool_free(s, s->q->bio_split_hook);
> +}
> +
> +static void bio_submit_split_endio(struct bio *bio, int error)
> +{
> +	struct closure *cl = bio->bi_private;
> +	struct bio_split_hook *s = container_of(cl, struct bio_split_hook, cl);
> +
> +	if (error)
> +		clear_bit(BIO_UPTODATE, &s->bio->bi_flags);
> +
> +	bio_put(bio);
> +	closure_put(cl);
> +}
> +
> +static void __bio_submit_split(struct closure *cl)
> +{
> +	struct bio_split_hook *s = container_of(cl, struct bio_split_hook, cl);
> +	struct bio *bio = s->bio, *n;
> +
> +	do {
> +		n = bio_split(bio, bio_max_sectors(bio),
> +			      GFP_NOIO, s->q->bio_split);
> +		if (!n)
> +			continue_at(cl, __bio_submit_split, bio_split_wq);
> +
> +		closure_get(cl);
> +		generic_make_request(n);
> +	} while (n != bio);
> +
> +	continue_at(cl, bio_submit_split_done, NULL);
> +}
> +
> +static int bio_submit_split(struct bio *bio)
> +{
> +	struct bio_split_hook *s;
> +	struct request_queue *q = bdev_get_queue(bio->bi_bdev);
> +
> +	if (!bio_has_data(bio) ||
> +	    !q ||
> +	    !q->bio_split_hook ||
> +	    bio_sectors(bio) <= bio_max_sectors(bio))
> +		return 0;
> +
> +	s = mempool_alloc(q->bio_split_hook, GFP_NOIO);
> +
> +	closure_init(&s->cl, NULL);
> +	s->bio		= bio;
> +	s->q		= q;
> +	s->bi_end_io	= bio->bi_end_io;
> +	s->bi_private	= bio->bi_private;
> +
> +	bio_get(bio);
> +	bio->bi_end_io	= bio_submit_split_endio;
> +	bio->bi_private	= &s->cl;
> +
> +	__bio_submit_split(&s->cl);
> +	return 1;
> +}
> +
>  /*
>   * Check whether this bio extends beyond the end of the device.
>   */
> @@ -1646,6 +1742,14 @@ void generic_make_request(struct bio *bio)
>  	 * it is non-NULL, then a make_request is active, and new requests
>  	 * should be added at the tail
>  	 */
> +
> +	/*
> +	 * If the device can't accept arbitrary sized bios, check if we
> +	 * need to split:
> +	 */
> +	if (bio_submit_split(bio))
> +		return;
> +
>  	if (current->bio_list) {
>  		bio_list_add(current->bio_list, bio);
>  		return;
> @@ -2892,11 +2996,18 @@ int __init blk_dev_init(void)
>  	if (!kblockd_workqueue)
>  		panic("Failed to create kblockd\n");
>  
> +	bio_split_wq = alloc_workqueue("bio_split", WQ_MEM_RECLAIM, 0);
> +	if (!bio_split_wq)
> +		panic("Failed to create bio_split wq\n");
> +
>  	request_cachep = kmem_cache_create("blkdev_requests",
>  			sizeof(struct request), 0, SLAB_PANIC, NULL);
>  
>  	blk_requestq_cachep = kmem_cache_create("blkdev_queue",
>  			sizeof(struct request_queue), 0, SLAB_PANIC, NULL);
>  
> +	bio_split_cache = kmem_cache_create("bio_split_hook",
> +			sizeof(struct bio_split_hook), 0, SLAB_PANIC, NULL);
> +
>  	return 0;
>  }
> diff --git a/fs/bio.c b/fs/bio.c
> index 781a8cf..360ac93 100644
> --- a/fs/bio.c
> +++ b/fs/bio.c
> @@ -428,6 +428,47 @@ inline int bio_phys_segments(struct request_queue *q, struct bio *bio)
>  }
>  EXPORT_SYMBOL(bio_phys_segments);
>  
> +unsigned __bio_max_sectors(struct bio *bio, struct block_device *bdev,
> +			   sector_t sector)
> +{
> +	unsigned ret = bio_sectors(bio);
> +	struct request_queue *q = bdev_get_queue(bdev);
> +	struct bio_vec *bv, *end = bio_iovec(bio) +
> +		min_t(int, bio_segments(bio), queue_max_segments(q));
> +
> +	struct bvec_merge_data bvm = {
> +		.bi_bdev	= bdev,
> +		.bi_sector	= sector,
> +		.bi_size	= 0,
> +		.bi_rw		= bio->bi_rw,
> +	};
> +
> +	if (bio_segments(bio) > queue_max_segments(q) ||
> +	    q->merge_bvec_fn) {
> +		ret = 0;
> +
> +		for (bv = bio_iovec(bio); bv < end; bv++) {
> +			if (q->merge_bvec_fn &&
> +			    q->merge_bvec_fn(q, &bvm, bv) < (int) bv->bv_len)
> +				break;
> +
> +			ret		+= bv->bv_len >> 9;
> +			bvm.bi_size	+= bv->bv_len;
> +		}
> +
> +		if (ret >= (BIO_MAX_PAGES * PAGE_SIZE) >> 9)
> +			return (BIO_MAX_PAGES * PAGE_SIZE) >> 9;
> +	}
> +
> +	ret = min(ret, queue_max_sectors(q));
> +
> +	WARN_ON(!ret);
> +	ret = max_t(int, ret, bio_iovec(bio)->bv_len >> 9);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(__bio_max_sectors);
> +
>  /**
>   * 	__bio_clone	-	clone a bio
>   * 	@bio: destination bio
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index db38175..79f8173 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -224,6 +224,13 @@ extern void bio_endio(struct bio *, int);
>  struct request_queue;
>  extern int bio_phys_segments(struct request_queue *, struct bio *);
>  
> +unsigned __bio_max_sectors(struct bio *, struct block_device *, sector_t);
> +
> +static inline unsigned bio_max_sectors(struct bio *bio)
> +{
> +	return __bio_max_sectors(bio, bio->bi_bdev, bio->bi_sector);
> +}
> +
>  extern void __bio_clone(struct bio *, struct bio *);
>  extern struct bio *bio_clone_bioset(struct bio *, gfp_t, struct bio_set *bs);
>  extern struct bio *bio_clone(struct bio *, gfp_t);
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 2aa2466..464adb7 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -399,6 +399,9 @@ struct request_queue {
>  	/* Throttle data */
>  	struct throtl_data *td;
>  #endif
> +
> +	mempool_t		*bio_split_hook;
> +	struct bio_set		*bio_split;
>  };
>  
>  #define QUEUE_FLAG_QUEUED	1	/* uses generic tag queueing */
> -- 
> 1.7.9.rc2
> 
> --
> dm-devel mailing list
> dm-devel@...hat.com
> https://www.redhat.com/mailman/listinfo/dm-devel
> 
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