Re: [RFC PATCH 27/28] sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage

[Trond Myklebust <trondmy@xxxxxxxxxxxxxxx>]

On Mar 16, 2023, at 12:24, David Howells <dhowells@xxxxxxxxxx> wrote:

Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote:

+ buf->bvec += 2;
+ buf->bvec[-2].bv_page = NULL;
+ buf->bvec[-1].bv_page = NULL;

NACK.

Can you elaborate?

Is it that you dislike allocating extra slots for protocol bits?  Or just that
the bvec[] is offset by 2?  Or some other reason?

1) This is code that is common to the client and the server. Why are we adding unused 3  bvec slots to every client RPC call?

2) It obfuscates the existence of these bvec slots.

3) knfsd may use splice_direct_to_actor() in order to avoid copying the page cache data into private buffers (it just takes a reference to the pages). Using MSG_SPLICE_PAGES will presumably require it to protect those pages against further writes while the socket is referencing them.

_________________________________

Trond Myklebust

Linux NFS client maintainer, Hammerspace

trond.myklebust@xxxxxxxxxxxxxxx