[PATCH 02/10] block: copy bip_max_vcnt vecs instead of bip_vcnt during clone

[Kanchan Joshi]

On 4/27/2024 12:33 PM, Christoph Hellwig wrote:
>> If bio_integrity_copy_user is used to process the meta buffer, bip_max_vcnt
>> is one greater than bip_vcnt. In this case bip_max_vcnt vecs needs to be
>> copied to cloned bip.
> Can you explain this a bit more?  The clone should only allocate what
> is actually used, so this leaves be a bit confused.
> 

Will expand the commit description.

Usually the meta buffer is pinned and used directly (say N bio vecs).
In case kernel has to make a copy (in bio_integrity_copy_user), it 
factors these N vecs in, and one extra for the bounce buffer.
So for read IO, bip_max_vcnt is N+1, while bip_vcnt is N.

The clone bio also needs to be aware of all N+1 vecs, so that we can 
copy the data from the bounce buffer to pinned user pages correctly 
during read-completion.