linux-stable.git/drivers/md/dm-snap.c, branch linux-5.14.y

dm snapshot: properly fix a crash when an origin has no snapshots

2021-05-25T20:19:58+00:00

If an origin target has no snapshots, o->split_boundary is set to 0.
This causes BUG_ON(sectors <= 0) in block/bio.c:bio_split().

Fix this by initializing chunk_size, and in turn split_boundary, to
rounddown_pow_of_two(UINT_MAX) -- the largest power of two that fits
into "unsigned" type.

Signed-off-by: Mikulas Patocka 
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer

dm snapshot: revert "fix a crash when an origin has no snapshots"

2021-05-25T20:16:47+00:00

Commit 7ee06ddc4038f936b0d4459d37a7d4d844fb03db ("dm snapshot: fix a
crash when an origin has no snapshots") introduced a regression in
snapshot merging - causing the lvm2 test lvcreate-cache-snapshot.sh
got stuck in an infinite loop.

Even though commit 7ee06ddc4038f936b0d4459d37a7d4d844fb03db was marked
for stable@ the stable team was notified to _not_ backport it.

Fixes: 7ee06ddc4038 ("dm snapshot: fix a crash when an origin has no snapshots")
Signed-off-by: Mikulas Patocka 
Signed-off-by: Mike Snitzer

dm snapshot: fix crash with transient storage and zero chunk size

2021-05-13T18:42:52+00:00

The following commands will crash the kernel:

modprobe brd rd_size=1048576
dmsetup create o --table "0 `blockdev --getsize /dev/ram0` snapshot-origin /dev/ram0"
dmsetup create s --table "0 `blockdev --getsize /dev/ram0` snapshot /dev/ram0 /dev/ram1 N 0"

The reason is that when we test for zero chunk size, we jump to the label
bad_read_metadata without setting the "r" variable. The function
snapshot_ctr destroys all the structures and then exits with "r == 0". The
kernel then crashes because it falsely believes that snapshot_ctr
succeeded.

In order to fix the bug, we set the variable "r" to -EINVAL.

Signed-off-by: Mikulas Patocka 
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer

dm snapshot: fix a crash when an origin has no snapshots

2021-05-10T15:11:36+00:00

If an origin target has no snapshots, o->split_boundary is set to 0.
This causes BUG_ON(sectors <= 0) in block/bio.c:bio_split().

Fix this by initializing chunk_size, and in turn split_boundary, to
rounddown_pow_of_two(UINT_MAX) -- the largest power of two that fits
into "unsigned" type.

Reported-by: Michael Tokarev 
Tested-by: Michael Tokarev 
Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka 
Signed-off-by: Mike Snitzer

dm: replace dm_vcalloc()

2021-04-19T17:13:26+00:00

Use kvcalloc or kvmalloc_array instead (depending whether zeroing is
useful).

Signed-off-by: Matthew Wilcox (Oracle) 
Signed-off-by: Mike Snitzer

dm snapshot: flush merged data before committing metadata

2021-01-06T23:28:34+00:00

If the origin device has a volatile write-back cache and the following
events occur:

1: After finishing merge operation of one set of exceptions,
   merge_callback() is invoked.
2: Update the metadata in COW device tracking the merge completion.
   This update to COW device is flushed cleanly.
3: System crashes and the origin device's cache where the recent
   merge was completed has not been flushed.

During the next cycle when we read the metadata from the COW device,
we will skip reading those metadata whose merge was completed in
step (1). This will lead to data loss/corruption.

To address this, flush the origin device post merge IO before
updating the metadata.

Cc: stable@vger.kernel.org
Signed-off-by: Akilesh Kailash 
Signed-off-by: Mike Snitzer

block: rename generic_make_request to submit_bio_noacct

2020-07-01T13:27:24+00:00

generic_make_request has always been very confusingly misnamed, so rename
it to submit_bio_noacct to make it clear that it is submit_bio minus
accounting and a few checks.

Signed-off-by: Christoph Hellwig 
Signed-off-by: Jens Axboe

dm snapshot: use true/false for bool variable

2020-01-07T17:07:17+00:00

Fixes coccicheck warning:

drivers/md/dm-snap.c:1064:3-18: WARNING: Assignment of 0/1 to bool variable
drivers/md/dm-snap.c:1152:1-16: WARNING: Assignment of 0/1 to bool variable
drivers/md/dm-snap.c:1317:1-16: WARNING: Assignment of 0/1 to bool variable

Reported-by: Hulk Robot 
Signed-off-by: zhengbin 
Signed-off-by: Mike Snitzer

dm snapshot: rework COW throttling to fix deadlock

2019-10-10T13:46:05+00:00

Commit 721b1d98fb517a ("dm snapshot: Fix excessive memory usage and
workqueue stalls") introduced a semaphore to limit the maximum number of
in-flight kcopyd (COW) jobs.

The implementation of this throttling mechanism is prone to a deadlock:

1. One or more threads write to the origin device causing COW, which is
   performed by kcopyd.

2. At some point some of these threads might reach the s->cow_count
   semaphore limit and block in down(&s->cow_count), holding a read lock
   on _origins_lock.

3. Someone tries to acquire a write lock on _origins_lock, e.g.,
   snapshot_ctr(), which blocks because the threads at step (2) already
   hold a read lock on it.

4. A COW operation completes and kcopyd runs dm-snapshot's completion
   callback, which ends up calling pending_complete().
   pending_complete() tries to resubmit any deferred origin bios. This
   requires acquiring a read lock on _origins_lock, which blocks.

   This happens because the read-write semaphore implementation gives
   priority to writers, meaning that as soon as a writer tries to enter
   the critical section, no readers will be allowed in, until all
   writers have completed their work.

   So, pending_complete() waits for the writer at step (3) to acquire
   and release the lock. This writer waits for the readers at step (2)
   to release the read lock and those readers wait for
   pending_complete() (the kcopyd thread) to signal the s->cow_count
   semaphore: DEADLOCK.

The above was thoroughly analyzed and documented by Nikos Tsironis as
part of his initial proposal for fixing this deadlock, see:
https://www.redhat.com/archives/dm-devel/2019-October/msg00001.html

Fix this deadlock by reworking COW throttling so that it waits without
holding any locks. Add a variable 'in_progress' that counts how many
kcopyd jobs are running. A function wait_for_in_progress() will sleep if
'in_progress' is over the limit. It drops _origins_lock in order to
avoid the deadlock.

Reported-by: Guruswamy Basavaiah 
Reported-by: Nikos Tsironis 
Reviewed-by: Nikos Tsironis 
Tested-by: Nikos Tsironis 
Fixes: 721b1d98fb51 ("dm snapshot: Fix excessive memory usage and workqueue stalls")
Cc: stable@vger.kernel.org # v5.0+
Depends-on: 4a3f111a73a8c ("dm snapshot: introduce account_start_copy() and account_end_copy()")
Signed-off-by: Mikulas Patocka 
Signed-off-by: Mike Snitzer

dm snapshot: introduce account_start_copy() and account_end_copy()

2019-10-10T13:45:48+00:00

This simple refactoring moves code for modifying the semaphore cow_count
into separate functions to prepare for changes that will extend these
methods to provide for a more sophisticated mechanism for COW
throttling.

Signed-off-by: Mikulas Patocka 
Reviewed-by: Nikos Tsironis 
Signed-off-by: Mike Snitzer