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 22:33:10 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
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 6/28/22 21:06, Al Viro wrote:
> 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

I don't see other iter types interesting in this context

> be to provide just such a primitive, wouldn't it?

A helper returning a page array sounds like overshot and waste of cycles
considering that it copies one bvec into another, and especially since
iov_iter_get_pages() parses only the first struct bio_vec and so returns
only 1 page at a time.

I can actually use for_each_bvec(), but still leaves updating the iter
from bvec_iter.

> 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...

It's the other way around, not all ITER_BVEC are managed but all users
asking to use managed frags (i.e. io_uring) should keep pages pinned and
provide ITER_BVEC. It's opt-in, both for users and protocols.

--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -66,9 +66,16 @@ struct msghdr {
  	};
  	bool		msg_control_is_user : 1;
  	bool		msg_get_inq : 1;/* return INQ after receive */
+	/*
+	 * The data pages are pinned and won't be released before ->msg_ubuf
+	 * is released. ->msg_iter should point to a bvec and ->msg_ubuf has
+	 * to be non-NULL.
+	 */
+	bool		msg_managed_data : 1;
  	unsigned int	msg_flags;	/* flags on received message */
  	__kernel_size_t	msg_controllen;	/* ancillary data buffer length */
  	struct kiocb	*msg_iocb;	/* ptr to iocb for async requests */
+	struct ubuf_info *msg_ubuf;
  };

The user sets ->msg_managed_data, then protocols find it and set
SKBFL_MANAGED_FRAG_REFS. If either of the steps didn't happen the
feature is not used.
The ->msg_managed_data part makes io_uring the only user, and io_uring
ensures pages are 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?

I hope a comment just above ->msg_managed_data should count as not quiet.

-- 
Pavel Begunkov