[LSF/MM/BPF ATTEND][LSF/MM/BPF TOPIC] Meta/Integrity/PI improvements

[Kanchan Joshi]

With respect to the current state of Meta/Block-integrity, there are
some missing pieces.
I can improve some of it. But not sure if I am up to speed on the
history behind the status quo.

Hence, this proposal to discuss the pieces.

Maybe people would like to discuss other points too, but I have the 
following:

- Generic user interface that user-space can use to exchange meta. A
new io_uring opcode IORING_OP_READ/WRITE_META - seems feasible for
direct IO. Buffered IO seems non-trivial as a relatively smaller meta
needs to be written into/read from the page cache. The related
metadata must also be written during the writeback (of data).


- Is there interest in filesystem leveraging the integrity capabilities 
that almost every enterprise SSD has.
Filesystems lacking checksumming abilities can still ask the SSD to do
it and be more robust.
And for BTRFS - there may be value in offloading the checksum to SSD.
Either to save the host CPU or to get more usable space (by not
writing the checksum tree). The mount option 'nodatasum' can turn off
the data checksumming, but more needs to be done to make the offload
work.

NVMe SSD can do the offload when the host sends the PRACT bit. But in
the driver, this is tied to global integrity disablement using
CONFIG_BLK_DEV_INTEGRITY.
So, the idea is to introduce a bio flag REQ_INTEGRITY_OFFLOAD
that the filesystem can send. The block-integrity and NVMe driver do
the rest to make the offload work.

- Currently, block integrity uses guard and ref tags but not application 
tags.
As per Martin's paper [*]:

"Work is in progress to implement support for the data
integrity extensions in btrfs, enabling the filesystem
to use the application tag."

I could not figure out more about the above effort. It will be good to
understand the progress/problems.

I hope to have the RFC (on the first two items) for better discussion.

[*] https://www.landley.net/kdocs/ols/2008/ols2008v2-pages-151-156.pdf