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

[Keith Busch <kbusch@kernel.org>]

On Mon, Apr 29, 2024 at 04:58:37PM +0530, Kanchan Joshi wrote:
> 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.

An earlier version added a field in the bip to point to the original
bvec from the user address. That extra field wouldn't be used in the far
majority of cases, so moving the user bvec to the end of the existing
bip_vec is a spatial optimization. The code may look a little more
confusing that way, but I think it's better than making the bip bigger.