APPLIED: [PATCH 0/1] [focal:linux, groovy:linux, hirsute:linux, impish:linux] block: return the correct bvec when checking for gaps

[Kleber Souza]

On 29.07.21 14:14, Tim Gardner wrote:
> Applied to focal:linux, hirsute:linux. Thanks.
> 
> groovy: Won't fix
> Impish: already upstream
> 
> Looks like this got missed in previous cycle, nor is there a Trello
> "Mailing List" card.

This patch was deliberately postponed because of the relatively high
risk for the point release kernels, as documented on the Trello
card.


Kleber

> 
> -rtg
> 
> On 7/6/21 11:14 AM, Tim Gardner wrote:
>> [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.
>>
>> 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.
>>
>