path: root/block/blk-mq-tag.c
AgeCommit message (Collapse)Author
2015-03-18blkmq: Fix NULL pointer deref when all reserved tags inSam Bradshaw
When allocating from the reserved tags pool, bt_get() is called with a NULL hctx. If all tags are in use, the hw queue is kicked to push out any pending IO, potentially freeing tags, and tag allocation is retried. The problem is that blk_mq_run_hw_queue() doesn't check for a NULL hctx. So we avoid it with a simple NULL hctx test. Tested by hammering mtip32xx with concurrent smartctl/hdparm. Signed-off-by: Sam Bradshaw <> Signed-off-by: Selvan Mani <> Fixes: b32232073e80 ("blk-mq: fix hang in bt_get()") Cc: Added appropriate comment. Signed-off-by: Jens Axboe <>
2015-02-11blk-mq: fix double-free in error pathTony Battersby
If the allocation of bt->bs fails, then bt->map can be freed twice, once in blk_mq_init_bitmap_tags() -> bt_alloc(), and once in blk_mq_init_bitmap_tags() -> bt_free(). Fix by setting the pointer to NULL after the first free. Cc: <> Signed-off-by: Tony Battersby <> Signed-off-by: Jens Axboe <>
2015-01-23blk-mq: add tag allocation policyShaohua Li
This is the blk-mq part to support tag allocation policy. The default allocation policy isn't changed (though it's not a strict FIFO). The new policy is round-robin for libata. But it's a try-best implementation. If multiple tasks are competing, the tags returned will be mixed (which is unavoidable even with !mq, as requests from different tasks can be mixed in queue) Cc: Jens Axboe <> Cc: Tejun Heo <> Cc: Christoph Hellwig <> Signed-off-by: Shaohua Li <> Signed-off-by: Jens Axboe <>
2015-01-14blk-mq: fix false negative out-of-tags conditionJens Axboe
The blk-mq tagging tries to maintain some locality between CPUs and the tags issued. The tags are split into groups of words, and the words may not be fully populated. When searching for a new free tag, blk-mq may look at partial words, hence it passes in an offset/size to find_next_zero_bit(). However, it does that wrong, the size must always be the full length of the number of tags in that word, otherwise we'll potentially miss some near the end. Another issue is when __bt_get() goes from one word set to the next. It bumps the index, but not the last_tag associated with the previous index. Bump that to be in the range of the new word. Finally, clean up __bt_get() and __bt_get_word() a bit and get rid of the goto in there, and the unnecessary 'wrap' variable. Signed-off-by: Jens Axboe <>
2014-12-31block: wake up waiters when a queue is marked dyingJens Axboe
If it's dying, we can't expect new request to complete and come in an wake up other tasks waiting for requests. So after we have marked it as dying, wake up everybody currently waiting for a request. Once they wake, they will retry their allocation and fail appropriately due to the state of the queue. Tested-by: Keith Busch <> Signed-off-by: Jens Axboe <>
2014-12-15Revert "blk-mq: Micro-optimize bt_get()"Jens Axboe
This reverts commit 52f7eb945f2ba62b324bb9ae16d945326a961dcf. The optimization is only really safe for a single queue, otherwise 'bs' and 'bt' can indeed change, and if we don't do a finish_wait() for each loop, we'll potentially change the wait structure and corrupt task wait list. Reported-by: Jan Kara <>
2014-12-13Merge branch 'for-3.19/core' of git:// Torvalds
Pull block driver core update from Jens Axboe: "This is the pull request for the core block IO changes for 3.19. Not a huge round this time, mostly lots of little good fixes: - Fix a bug in sysfs blktrace interface causing a NULL pointer dereference, when enabled/disabled through that API. From Arianna Avanzini. - Various updates/fixes/improvements for blk-mq: - A set of updates from Bart, mostly fixing buts in the tag handling. - Cleanup/code consolidation from Christoph. - Extend queue_rq API to be able to handle batching issues of IO requests. NVMe will utilize this shortly. From me. - A few tag and request handling updates from me. - Cleanup of the preempt handling for running queues from Paolo. - Prevent running of unmapped hardware queues from Ming Lei. - Move the kdump memory limiting check to be in the correct location, from Shaohua. - Initialize all software queues at init time from Takashi. This prevents a kobject warning when CPUs are brought online that weren't online when a queue was registered. - Single writeback fix for I_DIRTY clearing from Tejun. Queued with the core IO changes, since it's just a single fix. - Version X of the __bio_add_page() segment addition retry from Maurizio. Hope the Xth time is the charm. - Documentation fixup for IO scheduler merging from Jan. - Introduce (and use) generic IO stat accounting helpers for non-rq drivers, from Gu Zheng. - Kill off artificial limiting of max sectors in a request from Christoph" * 'for-3.19/core' of git:// (26 commits) bio: modify __bio_add_page() to accept pages that don't start a new segment blk-mq: Fix uninitialized kobject at CPU hotplugging blktrace: don't let the sysfs interface remove trace from running list blk-mq: Use all available hardware queues blk-mq: Micro-optimize bt_get() blk-mq: Fix a race between bt_clear_tag() and bt_get() blk-mq: Avoid that __bt_get_word() wraps multiple times blk-mq: Fix a use-after-free blk-mq: prevent unmapped hw queue from being scheduled blk-mq: re-check for available tags after running the hardware queue blk-mq: fix hang in bt_get() blk-mq: move the kdump check to blk_mq_alloc_tag_set blk-mq: cleanup tag free handling blk-mq: use 'nr_cpu_ids' as highest CPU ID count for hwq <-> cpu map blk: introduce generic io stat accounting help function blk-mq: handle the single queue case in blk_mq_hctx_next_cpu genhd: check for int overflow in disk_expand_part_tbl() blk-mq: add blk_mq_free_hctx_request() blk-mq: export blk_mq_free_request() blk-mq: use get_cpu/put_cpu instead of preempt_disable/preempt_enable ...
2014-12-09blk-mq: Micro-optimize bt_get()Bart Van Assche
Remove a superfluous finish_wait() call. Convert the two bt_wait_ptr() calls into a single call. Signed-off-by: Bart Van Assche <> Cc: Christoph Hellwig <> Cc: Robert Elliott <> Cc: Ming Lei <> Cc: Alexander Gordeev <> Signed-off-by: Jens Axboe <>
2014-12-09blk-mq: Fix a race between bt_clear_tag() and bt_get()Bart Van Assche
What we need is the following two guarantees: * Any thread that observes the effect of the test_and_set_bit() by __bt_get_word() also observes the preceding addition of 'current' to the appropriate wait list. This is guaranteed by the semantics of the spin_unlock() operation performed by prepare_and_wait(). Hence the conversion of test_and_set_bit_lock() into test_and_set_bit(). * The wait lists are examined by bt_clear() after the tag bit has been cleared. clear_bit_unlock() guarantees that any thread that observes that the bit has been cleared also observes the store operations preceding clear_bit_unlock(). However, clear_bit_unlock() does not prevent that the wait lists are examined before that the tag bit is cleared. Hence the addition of a memory barrier between clear_bit() and the wait list examination. Signed-off-by: Bart Van Assche <> Cc: Christoph Hellwig <> Cc: Robert Elliott <> Cc: Ming Lei <> Cc: Alexander Gordeev <> Cc: <> # v3.13+ Signed-off-by: Jens Axboe <>
2014-12-09blk-mq: Avoid that __bt_get_word() wraps multiple timesBart Van Assche
If __bt_get_word() is called with last_tag != 0, if the first find_next_zero_bit() fails, if after wrap-around the test_and_set_bit() call fails and find_next_zero_bit() succeeds, if the next test_and_set_bit() call fails and subsequently find_next_zero_bit() does not find a zero bit, then another wrap-around will occur. Avoid this by introducing an additional local variable. Signed-off-by: Bart Van Assche <> Cc: Christoph Hellwig <> Cc: Robert Elliott <> Cc: Ming Lei <> Cc: Alexander Gordeev <> Cc: <> # v3.13+ Signed-off-by: Jens Axboe <>
2014-12-08blk-mq: re-check for available tags after running the hardware queueJens Axboe
If we run out of tags and have to sleep, we run the hardware queue to kick pending IO into gear. During that run, we may have completed requests, so re-check if we have free tags before going to sleep. Signed-off-by: Jens Axboe <>
2014-12-08blk-mq: fix hang in bt_get()Bart Van Assche
Avoid that if there are fewer hardware queues than CPU threads that bt_get() can hang. The symptoms of the hang were as follows: * All tags allocated for a particular hardware queue. * (nr_tags) pending commands for that hardware queue. * No pending commands for the software queues associated with that hardware queue. Signed-off-by: Jens Axboe <>
2014-11-24blk-mq: cleanup tag free handlingJens Axboe
We only call __blk_mq_put_tag() and __blk_mq_put_reserved_tag() from blk_mq_put_tag(), so just inline the two calls instead of having them as separate functions. Signed-off-by: Jens Axboe <>
2014-11-12blk-mq: add blk_mq_unique_tag()Bart Van Assche
The queuecommand() callback functions in SCSI low-level drivers need to know which hardware context has been selected by the block layer. Since this information is not available in the request structure, and since passing the hctx pointer directly to the queuecommand callback function would require modification of all SCSI LLDs, add a function to the block layer that allows to query the hardware context index. Signed-off-by: Bart Van Assche <> Acked-by: Jens Axboe <> Reviewed-by: Sagi Grimberg <> Reviewed-by: Martin K. Petersen <> Signed-off-by: Christoph Hellwig <>
2014-10-07blk-mq: Make bt_clear_tag() easier to readBart Van Assche
Eliminate a backwards goto statement from bt_clear_tag(). Signed-off-by: Bart Van Assche <> Signed-off-by: Jens Axboe <>
2014-10-07blk-mq: fix potential hang if rolling wakeup depth is too highJens Axboe
We currently divide the queue depth by 4 as our batch wakeup count, but we split the wakeups over BT_WAIT_QUEUES number of wait queues. This defaults to 8. If the product of the resulting batch wake count and BT_WAIT_QUEUES is higher than the device queue depth, we can get into a situation where a task goes to sleep waiting for a request, but never gets woken up. Reported-by: Bart Van Assche <> Fixes: 4bb659b156996 Cc: Signed-off-by: Jens Axboe <>
2014-09-22blk-mq: fix and simplify tag iteration for the timeout handlerChristoph Hellwig
Don't do a kmalloc from timer to handle timeouts, chances are we could be under heavy load or similar and thus just miss out on the timeouts. Fortunately it is very easy to just iterate over all in use tags, and doing this properly actually cleans up the blk_mq_busy_iter API as well, and prepares us for the next patch by passing a reserved argument to the iterator. Signed-off-by: Christoph Hellwig <> Signed-off-by: Jens Axboe <>
2014-06-17blk-mq: bitmap tag: fix races in bt_get() functionAlexander Gordeev
This update fixes few issues in bt_get() function: - list_empty(&wait.task_list) check is not protected; - was_empty check is always true which results in *every* thread entering the loop resets bt_wait_state::wait_cnt counter rather than every bt->wake_cnt'th thread; - 'bt_wait_state::wait_cnt' counter update is redundant, since it also gets reset in bt_clear_tag() function; Cc: Christoph Hellwig <> Cc: Ming Lei <> Cc: Jens Axboe <> Signed-off-by: Alexander Gordeev <> Signed-off-by: Jens Axboe <>
2014-06-17blk-mq: bitmap tag: fix race on blk_mq_bitmap_tags::wake_cntAlexander Gordeev
This piece of code in bt_clear_tag() function is racy: bs = bt_wake_ptr(bt); if (bs && atomic_dec_and_test(&bs->wait_cnt)) { atomic_set(&bs->wait_cnt, bt->wake_cnt); wake_up(&bs->wait); } Since nothing prevents bt_wake_ptr() from returning the very same 'bs' address on multiple CPUs, the following scenario is possible: CPU1 CPU2 ---- ---- 0. bs = bt_wake_ptr(bt); bs = bt_wake_ptr(bt); 1. atomic_dec_and_test(&bs->wait_cnt) 2. atomic_dec_and_test(&bs->wait_cnt) 3. atomic_set(&bs->wait_cnt, bt->wake_cnt); If the decrement in [1] yields zero then for some amount of time the decrement in [2] results in a negative/overflow value, which is not expected. The follow-up assignment in [3] overwrites the invalid value with the batch value (and likely prevents the issue from being severe) which is still incorrect and should be a lesser. Cc: Ming Lei <> Cc: Jens Axboe <> Signed-off-by: Alexander Gordeev <> Signed-off-by: Jens Axboe <>
2014-06-17blk-mq: bitmap tag: fix races on shared ::wake_index fieldsAlexander Gordeev
Fix racy updates of shared blk_mq_bitmap_tags::wake_index and blk_mq_hw_ctx::wake_index fields. Cc: Ming Lei <> Signed-off-by: Alexander Gordeev <> Signed-off-by: Jens Axboe <>
2014-06-03blk-mq: fix schedule from atomic contextMing Lei
blk_mq_put_ctx() has to be called before io_schedule() in bt_get(). This patch fixes the problem by taking similar approach from percpu_ida allocation for the situation. Signed-off-by: Ming Lei <> Signed-off-by: Jens Axboe <>
2014-05-28blk-mq: add file comments and update copyright noticesJens Axboe
None of the blk-mq files have an explanatory comment at the top for what that particular file does. Add that and add appropriate copyright notices as well. Signed-off-by: Jens Axboe <>
2014-05-28blk-mq: remove blk_mq_wait_for_tagsChristoph Hellwig
The current logic for blocking tag allocation is rather confusing, as we first allocated and then free again a tag in blk_mq_wait_for_tags, just to attempt a non-blocking allocation and then repeat if someone else managed to grab the tag before us. Instead change blk_mq_alloc_request_pinned to simply do a blocking tag allocation itself and use the request we get back from it. Signed-off-by: Christoph Hellwig <> Signed-off-by: Jens Axboe <>
2014-05-23blk-mq: export blk_mq_tag_busy_iterSam Bradshaw
Export the blk-mq in-flight tag iterator for driver consumption. This is particularly useful in exception paths or SRSI where in-flight IOs need to be cancelled and/or reissued. The NVMe driver conversion will use this. Signed-off-by: Sam Bradshaw <> Signed-off-by: Matias Bjørling <> Signed-off-by: Jens Axboe <>
2014-05-20blk-mq: allow changing of queue depth through sysfsJens Axboe
For request_fn based devices, the block layer exports a 'nr_requests' file through sysfs to allow adjusting of queue depth on the fly. Currently this returns -EINVAL for blk-mq, since it's not wired up. Wire this up for blk-mq, so that it now also always dynamic adjustments of the allowed queue depth for any given block device managed by blk-mq. Signed-off-by: Jens Axboe <>
2014-05-19Merge branch 'for-3.16/blk-mq-tagging' into for-3.16/coreJens Axboe
Signed-off-by: Jens Axboe <> Conflicts: block/blk-mq-tag.c
2014-05-19blk-mq: move the cache friendly bitmap type of out blk-mq-tagJens Axboe
We will use it for the pending list in blk-mq core as well. Signed-off-by: Jens Axboe <>
2014-05-13blk-mq: improve support for shared tags mapsJens Axboe
This adds support for active queue tracking, meaning that the blk-mq tagging maintains a count of active users of a tag set. This allows us to maintain a notion of fairness between users, so that we can distribute the tag depth evenly without starving some users while allowing others to try unfair deep queues. If sharing of a tag set is detected, each hardware queue will track the depth of its own queue. And if this exceeds the total depth divided by the number of active queues, the user is actively throttled down. The active queue count is done lazily to avoid bouncing that data between submitter and completer. Each hardware queue gets marked active when it allocates its first tag, and gets marked inactive when 1) the last tag is cleared, and 2) the queue timeout grace period has passed. Signed-off-by: Jens Axboe <>
2014-05-10blk-mq: bitmap tag: cleanup blk_mq_init_tagsMing Lei
Both nr_cache and nr_tags arn't needed for bitmap tag anymore. Signed-off-by: Ming Lei <> Signed-off-by: Jens Axboe <>
2014-05-10blk-mq: bitmap tag: select random tag betweet 0 and (depth - 1)Ming Lei
The selected tag should be selected at random between 0 and (depth - 1) with probability 1/depth, instead between 0 and (depth - 2) with probability 1/(depth - 1). Signed-off-by: Ming Lei <> Signed-off-by: Jens Axboe <>
2014-05-10blk-mq: bitmap tag: remove barrier in bt_clear_tag()Ming Lei
The barrier isn't necessary because both atomic_dec_and_test() and wake_up() implicate one barrier. Signed-off-by: Ming Lei <> Signed-off-by: Jens Axboe <>
2014-05-10blk-mq: bitmap tag: use clear_bit_unlock in bt_clear_tag()Ming Lei
The unlock memory barrier need to order access to req in free path and clearing tag bit, otherwise either request free path may see a allocated request, or initialized request in allocate path might be modified by the ongoing free path. Signed-off-by: Ming Lei <> Signed-off-by: Jens Axboe <>
2014-05-09blk-mq: use sparser tag layout for lower queue depthJens Axboe
For best performance, spreading tags over multiple cachelines makes the tagging more efficient on multicore systems. But since we have 8 * sizeof(unsigned long) tags per cacheline, we don't always get a nice spread. Attempt to spread the tags over at least 4 cachelines, using fewer number of bits per unsigned long if we have to. This improves tagging performance in setups with 32-128 tags. For higher depths, the spread is the same as before (BITS_PER_LONG tags per cacheline). Signed-off-by: Jens Axboe <>
2014-05-09blk-mq: implement new and more efficient tagging schemeJens Axboe
blk-mq currently uses percpu_ida for tag allocation. But that only works well if the ratio between tag space and number of CPUs is sufficiently high. For most devices and systems, that is not the case. The end result if that we either only utilize the tag space partially, or we end up attempting to fully exhaust it and run into lots of lock contention with stealing between CPUs. This is not optimal. This new tagging scheme is a hybrid bitmap allocator. It uses two tricks to both be SMP friendly and allow full exhaustion of the space: 1) We cache the last allocated (or freed) tag on a per blk-mq software context basis. This allows us to limit the space we have to search. The key element here is not caching it in the shared tag structure, otherwise we end up dirtying more shared cache lines on each allocate/free operation. 2) The tag space is split into cache line sized groups, and each context will start off randomly in that space. Even up to full utilization of the space, this divides the tag users efficiently into cache line groups, avoiding dirtying the same one both between allocators and between allocator and freeer. This scheme shows drastically better behaviour, both on small tag spaces but on large ones as well. It has been tested extensively to show better performance for all the cases blk-mq cares about. Signed-off-by: Jens Axboe <>
2014-04-29blk-mq: fix waiting for reserved tagsJens Axboe
blk_mq_wait_for_tags() is only able to wait for "normal" tags, not reserved tags. Pass in which one we should attempt to get a tag for, so that waiting for reserved tags will work. Reserved tags are used for internal commands, which are usually serialized. Hence no waiting generally takes place, but we should ensure that it actually works if users need that functionality. Signed-off-by: Jens Axboe <>
2014-04-15blk-mq: split out tag initialization, support shared tagsChristoph Hellwig
Add a new blk_mq_tag_set structure that gets set up before we initialize the queue. A single blk_mq_tag_set structure can be shared by multiple queues. Signed-off-by: Christoph Hellwig <> Modular export of blk_mq_{alloc,free}_tagset added by me. Signed-off-by: Jens Axboe <>
2014-02-14Merge branch 'for-linus' of git:// Torvalds
Pull block IO fixes from Jens Axboe: "Second round of updates and fixes for 3.14-rc2. Most of this stuff has been queued up for a while. The notable exception is the blk-mq changes, which are naturally a bit more in flux still. The pull request contains: - Two bug fixes for the new immutable vecs, causing crashes with raid or swap. From Kent. - Various blk-mq tweaks and fixes from Christoph. A fix for integrity bio's from Nic. - A few bcache fixes from Kent and Darrick Wong. - xen-blk{front,back} fixes from David Vrabel, Matt Rushton, Nicolas Swenson, and Roger Pau Monne. - Fix for a vec miscount with integrity vectors from Martin. - Minor annotations or fixes from Masanari Iida and Rashika Kheria. - Tweak to null_blk to do more normal FIFO processing of requests from Shlomo Pongratz. - Elevator switching bypass fix from Tejun. - Softlockup in blkdev_issue_discard() fix when !CONFIG_PREEMPT from me" * 'for-linus' of git:// (31 commits) block: add cond_resched() to potentially long running ioctl discard loop xen-blkback: init persistent_purge_work work_struct blk-mq: pair blk_mq_start_request / blk_mq_requeue_request blk-mq: dont assume rq->errors is set when returning an error from ->queue_rq block: Fix cloning of discard/write same bios block: Fix type mismatch in ssize_t_blk_mq_tag_sysfs_show blk-mq: rework flush sequencing logic null_blk: use blk_complete_request and blk_mq_complete_request virtio_blk: use blk_mq_complete_request blk-mq: rework I/O completions fs: Add prototype declaration to appropriate header file include/linux/bio.h fs: Mark function as static in fs/bio-integrity.c block/null_blk: Fix completion processing from LIFO to FIFO block: Explicitly handle discard/write same segments block: Fix nr_vecs for inline integrity vectors blk-mq: Add bio_integrity setup to blk_mq_make_request blk-mq: initialize sg_reserved_size blk-mq: handle dma_drain_size blk-mq: divert __blk_put_request for MQ ops blk-mq: support at_head inserations for blk_execute_rq ...
2014-02-10block: Fix type mismatch in ssize_t_blk_mq_tag_sysfs_showMasanari Iida
cppcheck detected following format string mismatch. [blk-mq-tag.c:201]: (warning) %u in format string (no. 1) requires 'unsigned int' but the argument type is 'int'. Change "cpu" from int to unsigned int, because the cpu never become minus value. Signed-off-by: Masanari Iida <> Signed-off-by: Jens Axboe <>
2014-01-23percpu_ida: Make percpu_ida_alloc + callers accept task state bitmaskKent Overstreet
This patch changes percpu_ida_alloc() + callers to accept task state bitmask for prepare_to_wait() for code like target/iscsi that needs it for interruptible sleep, that is provided in a subsequent patch. It now expects TASK_UNINTERRUPTIBLE when the caller is able to sleep waiting for a new tag, or TASK_RUNNING when the caller cannot sleep, and is forced to return a negative value when no tags are available. v2 changes: - Include blk-mq + tcm_fc + vhost/scsi + target/iscsi changes - Drop signal_pending_state() call v3 changes: - Only call prepare_to_wait() + finish_wait() when != TASK_RUNNING (PeterZ) Reported-by: Linus Torvalds <> Cc: Linus Torvalds <> Cc: Ingo Molnar <> Cc: Peter Zijlstra <> Cc: Jens Axboe <> Signed-off-by: Kent Overstreet <> Cc: <> #3.12+ Signed-off-by: Nicholas Bellinger <>
2013-10-25blk-mq: new multi-queue block IO queueing mechanismJens Axboe
Linux currently has two models for block devices: - The classic request_fn based approach, where drivers use struct request units for IO. The block layer provides various helper functionalities to let drivers share code, things like tag management, timeout handling, queueing, etc. - The "stacked" approach, where a driver squeezes in between the block layer and IO submitter. Since this bypasses the IO stack, driver generally have to manage everything themselves. With drivers being written for new high IOPS devices, the classic request_fn based driver doesn't work well enough. The design dates back to when both SMP and high IOPS was rare. It has problems with scaling to bigger machines, and runs into scaling issues even on smaller machines when you have IOPS in the hundreds of thousands per device. The stacked approach is then most often selected as the model for the driver. But this means that everybody has to re-invent everything, and along with that we get all the problems again that the shared approach solved. This commit introduces blk-mq, block multi queue support. The design is centered around per-cpu queues for queueing IO, which then funnel down into x number of hardware submission queues. We might have a 1:1 mapping between the two, or it might be an N:M mapping. That all depends on what the hardware supports. blk-mq provides various helper functions, which include: - Scalable support for request tagging. Most devices need to be able to uniquely identify a request both in the driver and to the hardware. The tagging uses per-cpu caches for freed tags, to enable cache hot reuse. - Timeout handling without tracking request on a per-device basis. Basically the driver should be able to get a notification, if a request happens to fail. - Optional support for non 1:1 mappings between issue and submission queues. blk-mq can redirect IO completions to the desired location. - Support for per-request payloads. Drivers almost always need to associate a request structure with some driver private command structure. Drivers can tell blk-mq this at init time, and then any request handed to the driver will have the required size of memory associated with it. - Support for merging of IO, and plugging. The stacked model gets neither of these. Even for high IOPS devices, merging sequential IO reduces per-command overhead and thus increases bandwidth. For now, this is provided as a potential 3rd queueing model, with the hope being that, as it matures, it can replace both the classic and stacked model. That would get us back to having just 1 real model for block devices, leaving the stacked approach to dm/md devices (as it was originally intended). Contributions in this patch from the following people: Shaohua Li <> Alexander Gordeev <> Christoph Hellwig <> Mike Christie <> Matias Bjorling <> Jeff Moyer <> Acked-by: Christoph Hellwig <> Signed-off-by: Jens Axboe <>