linux-kernel - Re: [RFC net-next v3 05/29] net: bvec specific path in zerocopy_sg_from

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Date:   Tue, 28 Jun 2022 21:06:10 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     io-uring@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jens Axboe <axboe@...nel.dk>, kernel-team@...com
Subject: Re: [RFC net-next v3 05/29] net: bvec specific path in
 zerocopy_sg_from_iter

On Tue, Jun 28, 2022 at 07:56:27PM +0100, Pavel Begunkov wrote:
> Add an bvec specialised and optimised path in zerocopy_sg_from_iter.
> It'll be used later for {get,put}_page() optimisations.

If you need a variant that would not grab page references for ITER_BVEC
(and presumably other non-userland ones), the natural thing to do would
be to provide just such a primitive, wouldn't it?

The fun question here is by which paths ITER_BVEC can be passed to that
function and which all of them are currently guaranteed to hold the
underlying pages pinned...

And AFAICS you quietly assume that only ITER_BVEC ones will ever have that
"managed" flag of your set.  Or am I misreading the next patch in the
series?