netdev - Re: [RFC PATCH 08/53] netfs: Add rsize to netfs_io

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Date: Mon, 16 Oct 2023 17:19:51 +0100
From: David Howells <dhowells@...hat.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: dhowells@...hat.com, Steve French <smfrench@...il.com>,
    Matthew Wilcox <willy@...radead.org>,
    Marc Dionne <marc.dionne@...istor.com>,
    Paulo Alcantara <pc@...guebit.com>,
    Shyam Prasad N <sprasad@...rosoft.com>, Tom Talpey <tom@...pey.com>,
    Dominique Martinet <asmadeus@...ewreck.org>,
    Ilya Dryomov <idryomov@...il.com>,
    Christian Brauner <christian@...uner.io>,
    linux-afs@...ts.infradead.org, linux-cifs@...r.kernel.org,
    linux-nfs@...r.kernel.org, ceph-devel@...r.kernel.org,
    v9fs@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
    linux-mm@...ck.org, netdev@...r.kernel.org,
    linux-kernel@...r.kernel.org, linux-cachefs@...hat.com
Subject: Re: [RFC PATCH 08/53] netfs: Add rsize to netfs_io_request

Jeff Layton <jlayton@...nel.org> wrote:

> > +	rreq->rsize = 4 * 1024 * 1024;
> >  	return 0;
> ...
> > +	rreq->rsize = 1024 * 1024;
> > +
> 
> Holy magic numbers, batman! I think this deserves a comment that
> explains how you came up with these values.

Actually, that should be set to something like the object size for ceph.

> Also, do 9p and cifs not need this for some reason?

At this point, cifs doesn't use netfslib, so that's implemented in a later
patch in this series.

9p does need setting, but I haven't tested that yet.  It probably needs
setting to 1MiB as I think that's the maximum the 9p transport can handle.

But in the case of cifs, this is actually dynamic, depending on how many
credits we can obtain.  The same may be true of ceph, though I'm not entirely
clear on that as yet.

For afs, the maximum [rw]size the protocol supports is actually something like
281350422593565 (ie. (65535-28) * (2^32-1)) minus a few bytes, but that's
probably not a good idea.  I might be best setting it at something like 256KiB
as that's what OpenAFS uses.

David