** Description changed:
+ [Impact]
+
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")
+ An upstream patch fixes this bug: commit
+ c9c9762d4d44dcb1b2ba90cfb4122dc11ceebf31 ("block: return the correct
+ bvec when checking for gaps")
Please include this patch in any supported kernels that are 5.1 or
later.
+
+ [Test Plan]
+
+ stress-ng --sequential 8 --class io -t 5m --times
+
+ [Where problems could occur]
+
+ Different incorrect pages could be wriiten to disk.
+
+ [Other Info]
+
+ This patch has already been released in all [FGHI] Azure kernels.
** Also affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Changed in: linux (Ubuntu Focal)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Focal)
Status: New => In Progress
** Changed in: linux (Ubuntu Groovy)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Groovy)
Status: New => In Progress
** Changed in: linux (Ubuntu Hirsute)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Hirsute)
Status: New => In Progress
** Changed in: linux (Ubuntu Impish)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Impish)
Status: New => In Progress
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https://bugs.launchpad.net/bugs/1931497
Title:
Patch To Fix Bug in the Linux Block Layer Responsible For Merging
BIOs
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