From: NeilBrown <neilb@suse.com>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>, Jens Axboe <axboe@kernel.dk>,
"linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
linux-block@vger.kernel.org,
device-mapper development <dm-devel@redhat.com>,
Zdenek Kabelac <zkabelac@redhat.com>
Subject: Re: [dm-devel] new patchset to eliminate DM's use of BIOSET_NEED_RESCUER
Date: Thu, 23 Nov 2017 16:12:46 +1100 [thread overview]
Message-ID: <87o9ntzigh.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <alpine.LRH.2.02.1711221250090.21196@file01.intranet.prod.int.rdu2.redhat.com>
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On Wed, Nov 22 2017, Mikulas Patocka wrote:
> On Wed, 22 Nov 2017, NeilBrown wrote:
>
>> On Tue, Nov 21 2017, Mikulas Patocka wrote:
>>
>> > On Tue, 21 Nov 2017, Mike Snitzer wrote:
>> >
>> >> On Tue, Nov 21 2017 at 4:23pm -0500,
>> >> Mikulas Patocka <mpatocka@redhat.com> wrote:
>> >>
>> >> > This is not correct:
>> >> >
>> >> > 2206 static void dm_wq_work(struct work_struct *work)
>> >> > 2207 {
>> >> > 2208 struct mapped_device *md = container_of(work, struct mapped_device, work);
>> >> > 2209 struct bio *bio;
>> >> > 2210 int srcu_idx;
>> >> > 2211 struct dm_table *map;
>> >> > 2212
>> >> > 2213 if (!bio_list_empty(&md->rescued)) {
>> >> > 2214 struct bio_list list;
>> >> > 2215 spin_lock_irq(&md->deferred_lock);
>> >> > 2216 list = md->rescued;
>> >> > 2217 bio_list_init(&md->rescued);
>> >> > 2218 spin_unlock_irq(&md->deferred_lock);
>> >> > 2219 while ((bio = bio_list_pop(&list)))
>> >> > 2220 generic_make_request(bio);
>> >> > 2221 }
>> >> > 2222
>> >> > 2223 map = dm_get_live_table(md, &srcu_idx);
>> >> > 2224
>> >> > 2225 while (!test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags)) {
>> >> > 2226 spin_lock_irq(&md->deferred_lock);
>> >> > 2227 bio = bio_list_pop(&md->deferred);
>> >> > 2228 spin_unlock_irq(&md->deferred_lock);
>> >> > 2229
>> >> > 2230 if (!bio)
>> >> > 2231 break;
>> >> > 2232
>> >> > 2233 if (dm_request_based(md))
>> >> > 2234 generic_make_request(bio);
>> >> > 2235 else
>> >> > 2236 __split_and_process_bio(md, map, bio);
>> >> > 2237 }
>> >> > 2238
>> >> > 2239 dm_put_live_table(md, srcu_idx);
>> >> > 2240 }
>> >> >
>> >> > You can see that if we are in dm_wq_work in __split_and_process_bio, we
>> >> > will not process md->rescued list.
>> >>
>> >> Can you elaborate further? We cannot be "in dm_wq_work in
>> >> __split_and_process_bio" simultaneously. Do you mean as a side-effect
>> >> of scheduling away from __split_and_process_bio?
>> >>
>> >> The more detail you can share the better.
>> >
>> > Suppose this scenario:
>> >
>> > * dm_wq_work calls __split_and_process_bio
>> > * __split_and_process_bio eventually reaches the function snapshot_map
>> > * snapshot_map attempts to take the snapshot lock
>> >
>> > * the snapshot lock could be released only if some bios submitted by the
>> > snapshot driver to the underlying device complete
>> > * the bios submitted to the underlying device were already offloaded by
>> > some other task and they are waiting on the list md->rescued
>> > * the bios waiting on md->rescued are not processed, because dm_wq_work is
>> > blocked in snapshot_map (called from __split_and_process_bio)
>>
>> Yes, I think you are right.
>>
>> I think the solution is to get rid of the dm_offload() infrastructure
>> and make it not necessary.
>> i.e. discard my patches
>> dm: prepare to discontinue use of BIOSET_NEED_RESCUER
>> and
>> dm: revise 'rescue' strategy for bio-based bioset allocations
>>
>> And build on "dm: ensure bio submission follows a depth-first tree walk"
>> which was written after those and already makes dm_offload() less
>> important.
>>
>> Since that "depth-first" patch, every request to the dm device, after
>> the initial splitting, allocates just one dm_target_io structure, and
>> makes just one __map_bio() call, and so will behave exactly the way
>> generic_make_request() expects and copes with - thus avoiding awkward
>> dependencies and deadlocks. Except....
>>
>> a/ If any target defines ->num_write_bios() to return >1,
>> __clone_and_map_data_bio() will make multiple calls to alloc_tio()
>> and __map_bio(), which might need rescuing.
>> But no target defines num_write_bios, and none have since it was
>> removed from dm-cache 4.5 years ago.
>> Can we discard num_write_bios??
>>
>> b/ If any target sets any of num_{flush,discard,write_same,write_zeroes}_bios
>> to a value > 1, then __send_duplicate_bios() will also make multiple
>> calls to alloc_tio() and __map_bio().
>> Some do.
>> dm-cache-target: flush=2
>> dm-snap: flush=2
>> dm-stripe: discard, write_same, write_zeroes all set to 'stripes'.
>>
>> These will only be a problem if the second (or subsequent) alloc_tio()
>> blocks waiting for an earlier allocation to complete. This will only
>> be a problem if multiple threads are each trying to allocate multiple
>> dm_target_io from the same bioset at the same time.
>> This is rare and should be easier to address than the current
>> dm_offload() approach.
>> One possibility would be to copy the approach taken by
>> crypt_alloc_buffer() which needs to allocate multiple entries from a
>> mempool.
>> It first tries the with GFP_NOWAIT. If that fails it take a mutex and
>> tries with GFP_NOIO. This mean only one thread will try to allocate
>> multiple bios at once, so there can be no deadlock.
>>
>> Below are two RFC patches. The first removes num_write_bios.
>> The second is incomplete and makes a stab are allocating multiple bios
>> at once safely.
>> A third would be needed to remove dm_offload() etc... but I cannot quite
>> fit that in today - must be off.
>>
>> Thanks,
>> NeilBrown
>
> Another problem is this:
>
> struct bio *b = bio_clone_bioset(bio, GFP_NOIO, md->queue->bio_split);
> bio_advance(b, (bio_sectors(b) - ci.sector_count) << 9);
> bio_chain(b, bio);
>
> What if it blocks because the bioset is exhausted?
>
> The code basically builds a chain of bios of unlimited length (suppose for
> example a case when we are splitting on every sector boundary, so there
> will be one bio for every sector in the original bio), it could exhaust
> the bioset easily.
>
> It would be better to use mechanism from md-raid that chains all the
> sub-bios to the same master bio and doesn't create long chains of bios:
>
> if (max_sectors < bio_sectors(bio)) {
> struct bio *split = bio_split(bio, max_sectors,
> gfp, conf->bio_split);
> bio_chain(split, bio);
> generic_make_request(bio);
> bio = split;
> r1_bio->master_bio = bio;
> r1_bio->sectors = max_sectors;
> }
>
> Mikulas
Yes, you are right something like that would be better.
Also send_changing_extent_only allocates bios in a loop which can cause
problems.
I think we need to get __split_and_process_non_flush(), in all its
branches, to check if len is too large for a single request, and if it
is, create a clone for the prefix, attached that to the ci and map it,
advance the original bio, and call generic_make_request on it.
That shouldn't be too hard, but it is a change that would touch a few
places.
I'll see if I can write something.
Thanks,
NeilBrown
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next prev parent reply other threads:[~2017-11-23 5:12 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-18 4:38 [PATCH 00/13] block: assorted cleanup for bio splitting and cloning NeilBrown
2017-06-18 4:38 ` [PATCH 03/13] blk: make the bioset rescue_workqueue optional NeilBrown
2017-06-18 4:38 ` [PATCH 04/13] blk: use non-rescuing bioset for q->bio_split NeilBrown
2017-06-18 4:38 ` [PATCH 01/13] blk: remove bio_set arg from blk_queue_split() NeilBrown
2017-06-18 4:38 ` [PATCH 02/13] blk: replace bioset_create_nobvec() with a flags arg to bioset_create() NeilBrown
2017-06-18 4:38 ` [PATCH 06/13] rbd: use bio_clone_fast() instead of bio_clone() NeilBrown
2017-06-18 4:38 ` [PATCH 08/13] pktcdvd: " NeilBrown
2017-06-18 4:38 ` [PATCH 05/13] block: Improvements to bounce-buffer handling NeilBrown
2017-06-18 4:38 ` [PATCH 09/13] lightnvm/pblk-read: use bio_clone_fast() NeilBrown
2017-06-18 4:38 ` [PATCH 07/13] drbd: use bio_clone_fast() instead of bio_clone() NeilBrown
2017-06-18 4:38 ` [PATCH 11/13] bcache: use kmalloc to allocate bio in bch_data_verify() NeilBrown
2017-06-18 4:38 ` [PATCH 12/13] block: remove bio_clone() and all references NeilBrown
2017-06-18 4:38 ` [PATCH 13/13] block: don't check for BIO_MAX_PAGES in blk_bio_segment_split() NeilBrown
2017-06-18 4:38 ` [PATCH 10/13] xen-blkfront: remove bio splitting NeilBrown
2017-06-18 18:41 ` [PATCH 00/13] block: assorted cleanup for bio splitting and cloning Jens Axboe
2017-06-18 21:36 ` NeilBrown
2017-11-20 16:43 ` Mike Snitzer
2017-11-21 0:34 ` [dm-devel] " NeilBrown
2017-11-21 1:35 ` Mike Snitzer
2017-11-21 12:10 ` Mike Snitzer
2017-11-21 12:43 ` Mike Snitzer
2017-11-21 19:47 ` new patchset to eliminate DM's use of BIOSET_NEED_RESCUER [was: Re: [PATCH 00/13] block: assorted cleanup for bio splitting and cloning.] Mike Snitzer
2017-11-21 21:23 ` [dm-devel] " Mikulas Patocka
2017-11-21 22:51 ` new patchset to eliminate DM's use of BIOSET_NEED_RESCUER Mike Snitzer
2017-11-22 1:21 ` Mikulas Patocka
2017-11-22 2:32 ` Mike Snitzer
2017-11-22 4:00 ` [dm-devel] " NeilBrown
2017-11-22 4:28 ` Mike Snitzer
2017-11-22 21:18 ` Mike Snitzer
2017-11-22 18:24 ` [dm-devel] " Mikulas Patocka
2017-11-22 18:49 ` Mike Snitzer
2017-11-23 5:12 ` NeilBrown [this message]
2017-11-23 22:52 ` [PATCH] dm: use cloned bio as head, not remainder, in __split_and_process_bio() NeilBrown
2017-11-27 14:23 ` Mike Snitzer
2017-11-28 22:18 ` [dm-devel] " NeilBrown
2017-11-21 23:03 ` [dm-devel] new patchset to eliminate DM's use of BIOSET_NEED_RESCUER [was: Re: [PATCH 00/13] block: assorted cleanup for bio splitting and cloning.] NeilBrown
2017-11-21 19:44 ` [dm-devel] [PATCH 00/13] block: assorted cleanup for bio splitting and cloning NeilBrown
2017-11-21 19:50 ` Mike Snitzer
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