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.
