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SubjectRe: new patchset to eliminate DM's use of BIOSET_NEED_RESCUER
On Tue, Nov 21 2017 at 11:00pm -0500,
NeilBrown <neilb@suse.com> 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....

Yes, FYI I've also verified that even with just the "depth-first" patch
(and dm_offload disabled) the snapshot deadlock is fixed.

> 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??

Yes.

> 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.

Great.

> From: NeilBrown <neilb@suse.com>
> Date: Wed, 22 Nov 2017 14:25:18 +1100
> Subject: [PATCH] DM: remove num_write_bios target interface.
>
> No target provides num_write_bios and none has done
> since 2013.
> Having the possibility of num_write_bios > 1 complicates
> bio allocation.
> So remove the interface and assume there is only one bio
> needed.
> If a target ever needs more, it must provide a suitable
> bioset and allocate itself based on its particular needs.
>
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
> drivers/md/dm.c | 22 ++++------------------
> include/linux/device-mapper.h | 15 ---------------
> 2 files changed, 4 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index b20febd6cbc7..8c1a05609eea 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1323,27 +1323,13 @@ static int __clone_and_map_data_bio(struct clone_info *ci, struct dm_target *ti,
> {
> struct bio *bio = ci->bio;
> struct dm_target_io *tio;
> - unsigned target_bio_nr;
> - unsigned num_target_bios = 1;
> int r = 0;
>
> - /*
> - * Does the target want to receive duplicate copies of the bio?
> - */
> - if (bio_data_dir(bio) == WRITE && ti->num_write_bios)
> - num_target_bios = ti->num_write_bios(ti, bio);
> -
> - for (target_bio_nr = 0; target_bio_nr < num_target_bios; target_bio_nr++) {
> - tio = alloc_tio(ci, ti, target_bio_nr);
> - tio->len_ptr = len;
> - r = clone_bio(tio, bio, sector, *len);
> - if (r < 0) {
> - free_tio(tio);
> - break;
> - }
> + tio = alloc_tio(ci, ti, 0);
> + tio->len_ptr = len;
> + r = clone_bio(tio, bio, sector, *len);
> + if (r >= 0)
> __map_bio(tio);
> - }
> -

This bit is wrong, free_tio() is needed if clone_bio() fails. I can fix
it up though.

I'll work through your patches tomorrow.

Thanks,
Mike

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