0c9bed3698
commit f1ed3df20d2d223e0852cc4ac1f19bba869a7e3c upstream.
In many architectures loads may be reordered with older stores to
different locations. In the nvme driver the following two operations
could be reordered:
- Write shadow doorbell (dbbuf_db) into memory.
- Read EventIdx (dbbuf_ei) from memory.
This can result in a potential race condition between driver and VM host
processing requests (if given virtual NVMe controller has a support for
shadow doorbell). If that occurs, then the NVMe controller may decide to
wait for MMIO doorbell from guest operating system, and guest driver may
decide not to issue MMIO doorbell on any of subsequent commands.
This issue is purely timing-dependent one, so there is no easy way to
reproduce it. Currently the easiest known approach is to run "Oracle IO
Numbers" (orion) that is shipped with Oracle DB:
orion -run advanced -num_large 0 -size_small 8 -type rand -simulate \
concat -write 40 -duration 120 -matrix row -testname nvme_test
Where nvme_test is a .lun file that contains a list of NVMe block
devices to run test against. Limiting number of vCPUs assigned to given
VM instance seems to increase chances for this bug to occur. On test
environment with VM that got 4 NVMe drives and 1 vCPU assigned the
virtual NVMe controller hang could be observed within 10-20 minutes.
That correspond to about 400-500k IO operations processed (or about
100GB of IO read/writes).
Orion tool was used as a validation and set to run in a loop for 36
hours (equivalent of pushing 550M IO operations). No issues were
observed. That suggest that the patch fixes the issue.
Fixes: f9f38e33389c ("nvme: improve performance for virtual NVMe devices")
Signed-off-by: Michal Wnukowski <wnukowski@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
[hch: updated changelog and comment a bit]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.