netdev - Re: [PATCH net-next v5 04/16] ceph: Use sendmsg(MSG_SPLICE

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Date: Mon, 26 Jun 2023 18:01:56 +0100
From: David Howells <dhowells@...hat.com>
To: Ilya Dryomov <idryomov@...il.com>
Cc: dhowells@...hat.com, netdev@...r.kernel.org,
    Alexander Duyck <alexander.duyck@...il.com>,
    "David S. Miller" <davem@...emloft.net>,
    Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
    Paolo Abeni <pabeni@...hat.com>,
    Willem de Bruijn <willemdebruijn.kernel@...il.com>,
    David Ahern <dsahern@...nel.org>,
    Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
    linux-mm@...ck.org, linux-kernel@...r.kernel.org,
    Xiubo Li <xiubli@...hat.com>, Jeff Layton <jlayton@...nel.org>,
    ceph-devel@...r.kernel.org
Subject: Re: [PATCH net-next v5 04/16] ceph: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage()

Ilya Dryomov <idryomov@...il.com> wrote:

> > Btw, is it feasible to use con->v2.out_iter_sendpage to apply
> > MSG_SPLICE_PAGES to the iterator to be transmitted as a whole?  It seems
> > to be set depending on iterator type.
> 
> I'm not sure I understand what you mean by "transmitted as a whole".
> con->v2.out_iter_sendpage is set only when zerocopy is desired.  If the
> underlying data is not guaranteed to remain stable, zerocopy behavior
> is not safe.

I think I need to reinstate the per-page sendpage_ok() check here also -
though Al pointed out it isn't sufficiently exhaustive.  There are pages that
sendpage_ok() will return true on that you shouldn't be passing to sendpage().

I'll whip up a patch to partially revert this also.

David