linux-stable.git/drivers/md/bcache/writeback.h, branch linux-5.10.y

bcache: improve multithreaded bch_sectors_dirty_init()

2022-06-09T08:21:27+00:00

commit 4dc34ae1b45fe26e772a44379f936c72623dd407 upstream.

Commit b144e45fc576 ("bcache: make bch_sectors_dirty_init() to be
multithreaded") makes bch_sectors_dirty_init() to be much faster
when counting dirty sectors by iterating all dirty keys in the btree.
But it isn't in ideal shape yet, still can be improved.

This patch does the following changes to improve current parallel dirty
keys iteration on the btree,
- Add read lock to root node when multiple threads iterating the btree,
  to prevent the root node gets split by I/Os from other registered
  bcache devices.
- Remove local variable "char name[32]" and generate kernel thread name
  string directly when calling kthread_run().
- Allocate "struct bch_dirty_init_state state" directly on stack and
  avoid the unnecessary dynamic memory allocation for it.
- Decrease BCH_DIRTY_INIT_THRD_MAX from 64 to 12 which is enough indeed.
- Increase &state->started to count created kernel thread after it
  succeeds to create.
- When wait for all dirty key counting threads to finish, use
  wait_event() to replace wait_event_interruptible().

With the above changes, the code is more clear, and some potential error
conditions are avoided.

Fixes: b144e45fc576 ("bcache: make bch_sectors_dirty_init() to be multithreaded")
Signed-off-by: Coly Li 
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220524102336.10684-3-colyli@suse.de
Signed-off-by: Jens Axboe 
Signed-off-by: Greg Kroah-Hartman

bcache: fix overflow in offset_to_stripe()

2020-07-25T13:38:20+00:00

offset_to_stripe() returns the stripe number (in type unsigned int) from
an offset (in type uint64_t) by the following calculation,
	do_div(offset, d->stripe_size);
For large capacity backing device (e.g. 18TB) with small stripe size
(e.g. 4KB), the result is 4831838208 and exceeds UINT_MAX. The actual
returned value which caller receives is 536870912, due to the overflow.

Indeed in bcache_device_init(), bcache_device->nr_stripes is limited in
range [1, INT_MAX]. Therefore all valid stripe numbers in bcache are
in range [0, bcache_dev->nr_stripes - 1].

This patch adds a upper limition check in offset_to_stripe(): the max
valid stripe number should be less than bcache_device->nr_stripes. If
the calculated stripe number from do_div() is equal to or larger than
bcache_device->nr_stripe, -EINVAL will be returned. (Normally nr_stripes
is less than INT_MAX, exceeding upper limitation doesn't mean overflow,
therefore -EOVERFLOW is not used as error code.)

This patch also changes nr_stripes' type of struct bcache_device from
'unsigned int' to 'int', and return value type of offset_to_stripe()
from 'unsigned int' to 'int', to match their exact data ranges.

All locations where bcache_device->nr_stripes and offset_to_stripe() are
referenced also get updated for the above type change.

Reported-and-tested-by: Ken Raeburn 
Signed-off-by: Coly Li 
Cc: stable@vger.kernel.org
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1783075
Signed-off-by: Jens Axboe

bcache: make bch_sectors_dirty_init() to be multithreaded

2020-03-22T16:06:57+00:00

When attaching a cached device (a.k.a backing device) to a cache
device, bch_sectors_dirty_init() is called to count dirty sectors
and stripes (see what bcache_dev_sectors_dirty_add() does) on the
cache device.

The counting is done by a single thread recursive function
bch_btree_map_keys() to iterate all the bcache btree nodes.
If the btree has huge number of nodes, bch_sectors_dirty_init() will
take quite long time. In my testing, if the registering cache set has
a existed UUID which matches a already registered cached device, the
automatical attachment during the registration may take more than
55 minutes. This is too long for waiting the bcache to work in real
deployment.

Fortunately when bch_sectors_dirty_init() is called, no other thread
will access the btree yet, it is safe to do a read-only parallelized
dirty sectors counting by multiple threads.

This patch tries to create multiple threads, and each thread tries to
one-by-one count dirty sectors from the sub-tree indexed by a root
node key which the thread fetched. After the sub-tree is counted, the
counting thread will continue to fetch another root node key, until
the fetched key is NULL. How many threads in parallel depends on
the number of keys from the btree root node, and the number of online
CPU core. The thread number will be the less number but no more than
BCH_DIRTY_INIT_THRD_MAX. If there are only 2 keys in root node, it
can only be 2x times faster by this patch. But if there are 10 keys
in the root node, with this patch it can be 10x times faster.

Signed-off-by: Coly Li 
Cc: Christoph Hellwig 
Signed-off-by: Jens Axboe

bcache: never writeback a discard operation

2019-02-09T14:18:31+00:00

Some users see panics like the following when performing fstrim on a
bcached volume:

[  529.803060] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
[  530.183928] #PF error: [normal kernel read fault]
[  530.412392] PGD 8000001f42163067 P4D 8000001f42163067 PUD 1f42168067 PMD 0
[  530.750887] Oops: 0000 [#1] SMP PTI
[  530.920869] CPU: 10 PID: 4167 Comm: fstrim Kdump: loaded Not tainted 5.0.0-rc1+ #3
[  531.290204] Hardware name: HP ProLiant DL360 Gen9/ProLiant DL360 Gen9, BIOS P89 12/27/2015
[  531.693137] RIP: 0010:blk_queue_split+0x148/0x620
[  531.922205] Code: 60 38 89 55 a0 45 31 db 45 31 f6 45 31 c9 31 ff 89 4d 98 85 db 0f 84 7f 04 00 00 44 8b 6d 98 4c 89 ee 48 c1 e6 04 49 03 70 78 <8b> 46 08 44 8b 56 0c 48
8b 16 44 29 e0 39 d8 48 89 55 a8 0f 47 c3
[  532.838634] RSP: 0018:ffffb9b708df39b0 EFLAGS: 00010246
[  533.093571] RAX: 00000000ffffffff RBX: 0000000000046000 RCX: 0000000000000000
[  533.441865] RDX: 0000000000000200 RSI: 0000000000000000 RDI: 0000000000000000
[  533.789922] RBP: ffffb9b708df3a48 R08: ffff940d3b3fdd20 R09: 0000000000000000
[  534.137512] R10: ffffb9b708df3958 R11: 0000000000000000 R12: 0000000000000000
[  534.485329] R13: 0000000000000000 R14: 0000000000000000 R15: ffff940d39212020
[  534.833319] FS:  00007efec26e3840(0000) GS:ffff940d1f480000(0000) knlGS:0000000000000000
[  535.224098] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  535.504318] CR2: 0000000000000008 CR3: 0000001f4e256004 CR4: 00000000001606e0
[  535.851759] Call Trace:
[  535.970308]  ? mempool_alloc_slab+0x15/0x20
[  536.174152]  ? bch_data_insert+0x42/0xd0 [bcache]
[  536.403399]  blk_mq_make_request+0x97/0x4f0
[  536.607036]  generic_make_request+0x1e2/0x410
[  536.819164]  submit_bio+0x73/0x150
[  536.980168]  ? submit_bio+0x73/0x150
[  537.149731]  ? bio_associate_blkg_from_css+0x3b/0x60
[  537.391595]  ? _cond_resched+0x1a/0x50
[  537.573774]  submit_bio_wait+0x59/0x90
[  537.756105]  blkdev_issue_discard+0x80/0xd0
[  537.959590]  ext4_trim_fs+0x4a9/0x9e0
[  538.137636]  ? ext4_trim_fs+0x4a9/0x9e0
[  538.324087]  ext4_ioctl+0xea4/0x1530
[  538.497712]  ? _copy_to_user+0x2a/0x40
[  538.679632]  do_vfs_ioctl+0xa6/0x600
[  538.853127]  ? __do_sys_newfstat+0x44/0x70
[  539.051951]  ksys_ioctl+0x6d/0x80
[  539.212785]  __x64_sys_ioctl+0x1a/0x20
[  539.394918]  do_syscall_64+0x5a/0x110
[  539.568674]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

We have observed it where both:
1) LVM/devmapper is involved (bcache backing device is LVM volume) and
2) writeback cache is involved (bcache cache_mode is writeback)

On one machine, we can reliably reproduce it with:

 # echo writeback > /sys/block/bcache0/bcache/cache_mode
   (not sure whether above line is required)
 # mount /dev/bcache0 /test
 # for i in {0..10}; do
	file="$(mktemp /test/zero.XXX)"
	dd if=/dev/zero of="$file" bs=1M count=256
	sync
	rm $file
    done
  # fstrim -v /test

Observing this with tracepoints on, we see the following writes:

fstrim-18019 [022] .... 91107.302026: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0  DS 4260112 + 196352 hit 0 bypass 1
fstrim-18019 [022] .... 91107.302050: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0  DS 4456464 + 262144 hit 0 bypass 1
fstrim-18019 [022] .... 91107.302075: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0  DS 4718608 + 81920 hit 0 bypass 1
fstrim-18019 [022] .... 91107.302094: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0  DS 5324816 + 180224 hit 0 bypass 1
fstrim-18019 [022] .... 91107.302121: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0  DS 5505040 + 262144 hit 0 bypass 1
fstrim-18019 [022] .... 91107.302145: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0  DS 5767184 + 81920 hit 0 bypass 1
fstrim-18019 [022] .... 91107.308777: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0  DS 6373392 + 180224 hit 1 bypass 0


Note the final one has different hit/bypass flags.

This is because in should_writeback(), we were hitting a case where
the partial stripe condition was returning true and so
should_writeback() was returning true early.

If that hadn't been the case, it would have hit the would_skip test, and
as would_skip == s->iop.bypass == true, should_writeback() would have
returned false.

Looking at the git history from 'commit 72c270612bd3 ("bcache: Write out
full stripes")', it looks like the idea was to optimise for raid5/6:

       * If a stripe is already dirty, force writes to that stripe to
	 writeback mode - to help build up full stripes of dirty data

To fix this issue, make sure that should_writeback() on a discard op
never returns true.

More details of debugging:
https://www.spinics.net/lists/linux-bcache/msg06996.html

Previous reports:
 - https://bugzilla.kernel.org/show_bug.cgi?id=201051
 - https://bugzilla.kernel.org/show_bug.cgi?id=196103
 - https://www.spinics.net/lists/linux-bcache/msg06885.html

(Coly Li: minor modification to follow maximum 75 chars per line rule)

Cc: Kent Overstreet 
Cc: stable@vger.kernel.org
Fixes: 72c270612bd3 ("bcache: Write out full stripes")
Signed-off-by: Daniel Axtens 
Signed-off-by: Coly Li 
Signed-off-by: Jens Axboe

bcache: make cutoff_writeback and cutoff_writeback_sync tunable

2018-12-13T15:15:54+00:00

Currently the cutoff writeback and cutoff writeback sync thresholds are
defined by CUTOFF_WRITEBACK (40) and CUTOFF_WRITEBACK_SYNC (70) as
static values. Most of time these they work fine, but when people want
to do research on bcache writeback mode performance tuning, there is no
chance to modify the soft and hard cutoff writeback values.

This patch introduces two module parameters bch_cutoff_writeback_sync
and bch_cutoff_writeback which permit people to tune the values when
loading bcache.ko. If they are not specified by module loading, current
values CUTOFF_WRITEBACK_SYNC and CUTOFF_WRITEBACK will be used as
default and nothing changes.

When people want to tune this two values,
- cutoff_writeback can be set in range [1, 70]
- cutoff_writeback_sync can be set in range [1, 90]
- cutoff_writeback always <= cutoff_writeback_sync

The default values are strongly recommended to most of users for most of
workloads. Anyway, if people wants to take their own risk to do research
on new writeback cutoff tuning for their own workload, now they can make
it.

Signed-off-by: Coly Li 
Signed-off-by: Jens Axboe

bcache: option to automatically run gc thread after writeback

2018-12-13T15:15:54+00:00

The option gc_after_writeback is disabled by default, because garbage
collection will discard SSD data which drops cached data.

Echo 1 into /sys/fs/bcache//internal/gc_after_writeback will
enable this option, which wakes up gc thread when writeback accomplished
and all cached data is clean.

This option is helpful for people who cares writing performance more. In
heavy writing workload, all cached data can be clean only happens when
writeback thread cleans all cached data in I/O idle time. In such
situation a following gc running may help to shrink bcache B+ tree and
discard more clean data, which may be helpful for future writing
requests.

If you are not sure whether this is helpful for your own workload,
please leave it as disabled by default.

Signed-off-by: Coly Li 
Signed-off-by: Jens Axboe

bcache: add identifier names to arguments of function definitions

2018-08-11T21:46:41+00:00

There are many function definitions do not have identifier argument names,
scripts/checkpatch.pl complains warnings like this,

 WARNING: function definition argument 'struct bcache_device *' should
  also have an identifier name
  #16735: FILE: writeback.h:120:
  +void bch_sectors_dirty_init(struct bcache_device *);

This patch adds identifier argument names to all bcache function
definitions to fix such warnings.

Signed-off-by: Coly Li 
Reviewed: Shenghui Wang 
Signed-off-by: Jens Axboe

bcache: style fix to replace 'unsigned' by 'unsigned int'

2018-08-11T21:46:41+00:00

This patch fixes warning reported by checkpatch.pl by replacing 'unsigned'
with 'unsigned int'.

Signed-off-by: Coly Li 
Reviewed-by: Shenghui Wang 
Signed-off-by: Jens Axboe

bcache: simplify the calculation of the total amount of flash dirty data

2018-07-27T15:15:46+00:00

Currently we calculate the total amount of flash only devices dirty data
by adding the dirty data of each flash only device under registering
locker. It is very inefficient.

In this patch, we add a member flash_dev_dirty_sectors in struct cache_set
to record the total amount of flash only devices dirty data in real time,
so we didn't need to calculate the total amount of dirty data any more.

Signed-off-by: Tang Junhui 
Signed-off-by: Coly Li 
Signed-off-by: Jens Axboe

bcache: Fix indentation

2018-03-19T02:15:20+00:00

This patch avoids that smatch complains about inconsistent indentation.

Signed-off-by: Bart Van Assche 
Reviewed-by: Michael Lyle 
Reviewed-by: Coly Li 
Signed-off-by: Jens Axboe