[dm-crypt] [BISECT dbba42d8] dm: regression: large bio splits fail with dm-crypt under bcache in dm_make_request()

Eric Wheeler bcache at lists.ewheeler.net
Tue May 24 21:35:10 CEST 2016


[ New subject after bisect:
  Was: bcache gets stuck flushing writeback cache when used in 
       combination with LUKS/dm-crypt and non-default bucket size ]


Mikulas, Mike, please see below when you have a minute.  This bisect found 
a commit with your signoffs.  This affects all kernels >= v4.4.

Should the commit be reverted, or is there a better fix?  

Intentional top post, more below:

On Sun, 22 May 2016, James Johnston wrote:

> > On Fri, 20 May 2016, James Johnston wrote:
> > 
> > > > On Mon, 16 May 2016, Tim Small wrote:
> > > >
> > > > > On 08/05/16 19:39, James Johnston wrote:
> > > > > > I've run into a problem where the bcache writeback cache can't be flushed to
> > > > > > disk when the backing device is a LUKS / dm-crypt device and the cache set has
> > > > > > a non-default bucket size.  Basically, only a few megabytes will be flushed to
> > > > > > disk, and then it gets stuck.  Stuck means that the bcache writeback task
> > > > > > thrashes the disk by constantly reading hundreds of MB/second from the cache set
> > > > > > in an infinite loop, while not actually progressing (dirty_data never decreases
> > > > > > beyond a certain point).
> > > > >
> > > > > > [...]
> > > > >
> > > > > > The situation is basically unrecoverable as far as I can tell: if you attempt
> > > > > > to detach the cache set then the cache set disk gets thrashed extra-hard
> > > > > > forever, and it's impossible to actually get the cache set detached.  The only
> > > > > > solution seems to be to back up the data and destroy the volume...
> > > > >
> > > > > You can boot an older kernel to flush the device without destroying it
> > > > > (I'm guessing that's because older kernels split down the big requests
> > > > > which are failing on the 4.4 kernel).  Once flushed you could put the
> > > > > cache into writethrough mode, or use a smaller bucket size.
> > > >
> > > > Indeed, can someone test 4.1.y and see if the problem persists with a 2M
> > > > bucket size?  (If someone has already tested 4.1, then appologies as I've
> > > > not yet seen that report.)
> > > >
> > > > If 4.1 works, then I think a bisect is in order.  Such a bisect would at
> > > > least highlight the problem and might indicate a (hopefully trivial) fix.
> > >
> > > To help narrow this down, I tested the following generic pre-compiled mainline kernels
> > > on Ubuntu 15.10:
> > >
> > >  * WORKS:  http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.3.6-wily/
> > >  * DOES NOT WORK:  http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.4-rc1+cod1-wily/
> > >
> > > I also tried the default & latest distribution-provided 4.2 kernel.  It worked.
> > > This one also worked:
> > >
> > >  * WORKS:  http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.2.8-wily/
> > >
> > > So it seems to me that it is a regression from 4.3.6 kernel to any 4.4 kernel.  That
> > > should help save time with bisection...
> > 
> > Below is the patchlist for md and block that might help with a place to
> > start.  Are there any other places in the Linux tree where we should watch
> > for changes?
> > 
> > I'm wondering if it might be in dm-4.4-changes since this is dm-crypt
> > related, but it could be ac322de which was quite large.
> > 
> > James or Tim,
> > 
> > Can you try building ac322de?  If that produces the problem, then there
> > are only 3 more to try (unless this was actually a problem in 4.3 which
> > was fixed in 4.3.y, but hopefully that isn't so).
> > 
> > ccf21b6 is probably the next to test to rule out neil's big md patch,
> > which Linus abreviated in the commit log so it must be quite long.  OTOH,
> > if dm-4.4-changes works, then I'm not sure what commit might produce the
> > problem because the rest are not obviously relevant to the issue that are
> > more recent. 
> 
> So I decided to go ahead and bisect it today.  Looks like the bad commit is
> this one.  The commit prior flushed the bcache writeback cache without
> incident; this one does not and I guess caused this bcache regression.
> (FWIW ac322de came up during bisection, and tested good.)
> 
> johnstonj at kernel-build:~/linux$ git bisect bad
> dbba42d8a9ebddcc1c1412e8457f79f3cb6ef6e7 is the first bad commit
> commit dbba42d8a9ebddcc1c1412e8457f79f3cb6ef6e7
> Author: Mikulas Patocka <mpatocka at redhat.com>
> Date:   Wed Oct 21 16:34:20 2015 -0400
> 
>     dm: eliminate unused "bioset" process for each bio-based DM device
> 
>     Commit 54efd50bfd873e2dbf784e0b21a8027ba4299a3e ("block: make
>     generic_make_request handle arbitrarily sized bios") makes it possible
>     for block devices to process large bios.  In doing so that commit
>     allocates a new queue->bio_split bioset for each block device, this
>     bioset is used for allocating bios when the driver needs to split large
>     bios.
> 
>     Each bioset allocates a workqueue process, thus the above commit
>     increases the number of processes allocated per block device.
> 
>     DM doesn't need the queue->bio_split bioset, thus we can deallocate it.
>     This reduces the number of allocated processes per bio-based DM device
>     from 3 to 2.  Also remove the call to blk_queue_split(), it is not
>     needed because DM does its own splitting.
> 
>     Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
>     Signed-off-by: Mike Snitzer <snitzer at redhat.com>
> 
> The patch for this commit is very brief; reproduced here:
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 9555843..64b50b7 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1763,8 +1763,6 @@ static void dm_make_request(struct request_queue *q, struct bio *bio)
> 
>         map = dm_get_live_table(md, &srcu_idx);
> 
> -       blk_queue_split(q, &bio, q->bio_split);
> -
>         generic_start_io_acct(rw, bio_sectors(bio), &dm_disk(md)->part0);
> 
>         /* if we're suspended, we have to queue this io for later */
> @@ -2792,6 +2790,12 @@ int dm_setup_md_queue(struct mapped_device *md)
>         case DM_TYPE_BIO_BASED:
>                 dm_init_old_md_queue(md);
>                 blk_queue_make_request(md->queue, dm_make_request);
> +               /*
> +                * DM handles splitting bios as needed.  Free the bio_split bioset
> +                * since it won't be used (saves 1 process per bio-based DM device).
> +                */
> +               bioset_free(md->queue->bio_split);
> +               md->queue->bio_split = NULL;
>                 break;
>         }
> 
> Here is the bisect log:
> 
> johnstonj at kernel-build:~/linux$ git bisect log
> git bisect start
> # good: [6a13feb9c82803e2b815eca72fa7a9f5561d7861] Linux 4.3
> git bisect good 6a13feb9c82803e2b815eca72fa7a9f5561d7861
> # bad: [8005c49d9aea74d382f474ce11afbbc7d7130bec] Linux 4.4-rc1
> git bisect bad 8005c49d9aea74d382f474ce11afbbc7d7130bec
> # bad: [118c216e16c5ccb028cd03a0dcd56d17a07ff8d7] Merge tag 'staging-4.4-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging
> git bisect bad 118c216e16c5ccb028cd03a0dcd56d17a07ff8d7
> # good: [e627078a0cbdc0c391efeb5a2c4eb287328fd633] Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux
> git bisect good e627078a0cbdc0c391efeb5a2c4eb287328fd633
> # good: [c17c6da659571a115c7b4983da6c6ac464317c34] staging: wilc1000: rename pfScanResult of struct scan_attr
> git bisect good c17c6da659571a115c7b4983da6c6ac464317c34
> # good: [7bdb7d554e0e433b92b63f3472523cc3067f8ab4] Staging: rtl8192u: ieee80211: corrected indent
> git bisect good 7bdb7d554e0e433b92b63f3472523cc3067f8ab4
> # good: [ac322de6bf5416cb145b58599297b8be73cd86ac] Merge tag 'md/4.4' of git://neil.brown.name/md
> git bisect good ac322de6bf5416cb145b58599297b8be73cd86ac
> # good: [a4d8e93c3182a54d8d21a4d1cec6538ae1be9e16] Merge tag 'usb-for-v4.4' of git://git.kernel.org/pub/scm/linux/kernel/git/balbi/usb into usb-next
> git bisect good a4d8e93c3182a54d8d21a4d1cec6538ae1be9e16
> # good: [4f56f3fdca43c9a18339b6e0c3b1aa2f57f6d0b0] serial: 8250: Tolerate clock variance for max baud rate
> git bisect good 4f56f3fdca43c9a18339b6e0c3b1aa2f57f6d0b0
> # good: [e052c6d15c61cc4caff2f06cbca72b183da9f15e] tty: Use unbound workqueue for all input workers
> git bisect good e052c6d15c61cc4caff2f06cbca72b183da9f15e
> # good: [b9ca0c948c921e960006aaf319a29c004917cdf6] uwb: neh: Use setup_timer
> git bisect good b9ca0c948c921e960006aaf319a29c004917cdf6
> # bad: [aad9ae4550755edc020b5c511a8b54f0104b2f47] dm switch: simplify conditional in alloc_region_table()
> git bisect bad aad9ae4550755edc020b5c511a8b54f0104b2f47
> # good: [a3d939ae7b5f82688a6d3450f95286eaea338328] dm: convert ffs to __ffs
> git bisect good a3d939ae7b5f82688a6d3450f95286eaea338328
> # bad: [00272c854ee17b804ce81ef706f611dac17f4f89] dm linear: remove redundant target name from error messages
> git bisect bad 00272c854ee17b804ce81ef706f611dac17f4f89
> # bad: [4c7da06f5a780bbf44ebd7547789e48536d0a823] dm persistent data: eliminate unnecessary return values
> git bisect bad 4c7da06f5a780bbf44ebd7547789e48536d0a823
> # bad: [dbba42d8a9ebddcc1c1412e8457f79f3cb6ef6e7] dm: eliminate unused "bioset" process for each bio-based DM device
> git bisect bad dbba42d8a9ebddcc1c1412e8457f79f3cb6ef6e7
> # first bad commit: [dbba42d8a9ebddcc1c1412e8457f79f3cb6ef6e7] dm: eliminate unused "bioset" process for each bio-based DM device
> 
> Commands used for testing:
> 
> # Make cache set
> make-bcache --bucket 2M -C /dev/sdb
> # Set up backing device crypto
> cryptsetup luksFormat /dev/sdc
> cryptsetup open --type luks /dev/sdc backCrypt
> # Make backing device & enable writeback
> make-bcache -B /dev/mapper/backCrypt
> bcache-super-show /dev/sdb | grep cset.uuid | cut -f 3 > /sys/block/bcache0/bcache/attach
> echo writeback > /sys/block/bcache0/bcache/cache_mode
> 
> # KILL SEQUENCE
> 
> cd /sys/block/bcache0/bcache
> echo 0 > sequential_cutoff
> # Verify that the cache is attached (i.e. does not say "no cache")
> cat state
> dd if=/dev/urandom of=/dev/bcache0 bs=1M count=250
> cat dirty_data
> cat state
> # Next line causes severe disk thrashing and failure to flush writeback cache
> # on bad commits.
> echo 1 > detach
> cat dirty_data
> cat state
> 
> Hope this provides some insight into the problem...
> 
> James
> 
> 
> 

--
Eric Wheeler


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