Hi!
This patch introduces helpers of 'mp_bvec_iter_*' for multi-page bvec support.
The introduced helpers treate one bvec as real multi-page segment, which may include more than one pages.
The existed helpers of bvec_iter_* are interfaces for supporting current bvec iterator which is thought as single-page by drivers, fs, dm and etc. These introduced helpers will build single-page bvec in flight, so this way won't break current bio/bvec users, which needn't any change.
I don't understand why we have this in 4.19-stable. I don't see followup patches needing it, and it does not claim to fix a bug.
There is some more context on this at: https://lore.kernel.org/linux-block/YXweJ00CVsDLCI7b@google.com/T/#u and https://lore.kernel.org/stable/YYVZBuDaWBKT3vOS@google.com/T/#u
Thank you! Pavel