** Changed in: linux-azure (Ubuntu Hirsute)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1931497
Title:
Patch To Fix Bug in the Linux Block Layer Responsible For Merging
BIOs
Status in linux-azure package in Ubuntu:
New
Status in linux-azure source package in Focal:
Fix Committed
Status in linux-azure source package in Groovy:
Fix Committed
Status in linux-azure source package in Hirsute:
Fix Committed
Status in linux-azure source package in Impish:
New
Bug description:
There is a bug in the Linux block layer responsible for merging BIOs
that go across the page boundary. This bug was introduced in Linux 5.1
when the block layer BIO page tracking is enhanced to support multiple
pages.
Without this patch, data corruption can occur. The change to the
kernel block layer in Linux 5.1 changes the way multiple pages are
merged to a single block I/O descriptor, and how contiguous block I/O
descriptors are merged with previous descriptors.
If contiguous block I/O requests cross a page boundary of 4k, defined
by the hv_storvsc driver, the new block merge process can create two
pages of block I/O requests (the latter page with an offset) that
refer to the same physical sector on disk. This page list is then
assembled for the SCSI generic driver.
In the above scenario, when the block I/O request sizes are 512 bytes,
the Azure LIS driver (hv_storvsc module) is not able to correctly
parse the page array from the SCSI generic driver due to this bug in
Linux block layer and creates a potential overflow of offset I/O
requests and corruption of data on disk.
Mitigation of data loss is proven with filesystems with block size 4k.
When block I/O requests are of sizes 4k or multiples of 4k, they are
the page aligned in the memory and are not affected by the block I/O
merging algorithm introduced in Linux 5.1. Most modern file systems
use 4k I/O block size by default, thus mitigating this problem.
We have submitted a patch to Linux mainline kernel to fix this:
https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-
block.git/commit/?h=for-
next&id=c9c9762d4d44dcb1b2ba90cfb4122dc11ceebf31
c9c9762d4d44 ("block: return the correct bvec when checking for gaps")
Please include this patch in any supported kernels that are 5.1 or
later.
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