Messages in this thread | | | Date | Fri, 26 Aug 2022 20:32:44 +0100 | From | Al Viro <> | Subject | Re: [PATCH v3 1/1] nvmet-tcp: Don't kmap() pages which can't come from HIGHMEM |
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On Fri, Aug 26, 2022 at 08:16:59PM +0200, Fabio M. De Francesco wrote:
> As you may have already read, I'm so new to kernel development that I still > know very little about many subsystems and drivers. I am not currently > able to tell the difference between BVEC and KVEC. I could probably try to > switch from one to the other (after learning from other code), however I won't > be able to explain in the commit message why users should better use BVEC in > this case.
struct kvec: pairs of form <kernel address, length> struct bio_vec: triples of form <page, offset, length>
Either is a way to refer to a chunk of memory; the former obviously has it already mapped (you don't get kernel addresses otherwise), the latter doesn't need to.
iov_iter instances might be backed by different things, including arrays of kvec (iov_iter_kvec() constructs such) and arrays of bio_vec (iov_iter_bvec() is the constructor for those).
iov_iter primitives (copy_to_iter/copy_from_iter/copy_page_to_iter/etc.) work with either variant - they look at the flavour and act accordingly.
ITER_BVEC ones tend to do that kmap_local_page() + copy + kunmap_local(). ITER_KVEC obviously use memcpy() for copying and that's it.
If you need e.g. to send some subranges of some pages you could kmap them, form kvec array, make msg.msg_iter a KVEC-backed iterator over those and feed it to sendmsg(). Or you could take a bio_vec array instead, make msg.msg_iter a BVEC-backed iterator over that and feed to sendmsg().
The difference is, in the latter case kmap_local() will be done on demand *inside* ->sendmsg() instance, when it gets around to copying some data from the source and calls something like csum_and_copy_from_iter() or whichever primitive it chooses to use.
Why bother with mapping the damn thing in the caller and having it pinned all along whatever ->sendmsg() you end up calling? Just give it page/offset/length instead of address/length and let lib/iov_iter.c do the right thing...
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