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Date:   Thu, 23 Nov 2017 16:12:46 +1100
From:   NeilBrown <neilb@...e.com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Mike Snitzer <snitzer@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org,
        device-mapper development <dm-devel@...hat.com>,
        Zdenek Kabelac <zkabelac@...hat.com>
Subject: Re: [dm-devel] new patchset to eliminate DM's use of BIOSET_NEED_RESCUER

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@...hat.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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