Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Al Viro <viro@zeniv.linux.org.uk>, Sagi Grimberg <sagi@grimberg.me>
Cc:     Christoph Hellwig <hch@lst.de>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        James Smart <james.smart@broadcom.com>,
        Ira Weiny <ira.weiny@intel.com>,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@intel.com>,
        linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
        Chaitanya Kulkarni <chaitanyak@nvidia.com>,
        Keith Busch <kbusch@kernel.org>
Subject: Re: [PATCH v3 1/1] nvmet-tcp: Don't kmap() pages which can't come from HIGHMEM
Date:   Tue, 30 Aug 2022 22:06:54 +0200
Message-ID: <1924196.usQuhbGJ8B@opensuse>
In-Reply-To: <Ywkf3NJQ7/8SSV+e@ZenIV>
References: <20220822142438.5954-1-fmdefrancesco@gmail.com> <2887364.VdNmn5OnKV@opensuse> <Ywkf3NJQ7/8SSV+e@ZenIV>
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="utf-8"
Precedence: bulk

On venerd=C3=AC 26 agosto 2022 21:32:44 CEST Al Viro wrote:
> On Fri, Aug 26, 2022 at 08:16:59PM +0200, Fabio M. De Francesco wrote:
> > As you may have already read, I'm so new to kernel development that I=20
still
> > know very little about many subsystems and drivers. I am not currently
> > able to tell the difference between BVEC and KVEC. I could probably try=
 to
> > switch from one to the other (after learning from other code), however I
> > won't be able to explain in the commit message why users should better =
use
> > BVEC in this case.
>=20
> struct kvec: pairs of form <kernel address, length>
> struct bio_vec: triples of form <page, offset, length>
>=20
> Either is a way to refer to a chunk of memory; the former obviously has it
> already mapped (you don't get kernel addresses otherwise), the latter=20
doesn't
> need to.
>=20
> iov_iter instances might be backed by different things, including
> arrays of kvec (iov_iter_kvec() constructs such) and arrays of
> bio_vec (iov_iter_bvec() is the constructor for those).
>=20
> iov_iter primitives (copy_to_iter/copy_from_iter/copy_page_to_iter/etc.)
> work with either variant - they look at the flavour and act accordingly.
>=20
> ITER_BVEC ones tend to do that kmap_local_page() + copy + kunmap_local().
> ITER_KVEC obviously use memcpy() for copying and that's it.
>=20
> If you need e.g. to send some subranges of some pages you could kmap them,
> form kvec array, make msg.msg_iter a KVEC-backed iterator over those and
> feed it to sendmsg().  Or you could take a bio_vec array instead, make
> msg.msg_iter a BVEC-backed iterator over that and feed to sendmsg().
>=20
> The difference is, in the latter case kmap_local() will be done on demand
> *inside* ->sendmsg() instance, when it gets around to copying some data
> from the source and calls something like csum_and_copy_from_iter() or
> whichever primitive it chooses to use.
>=20
> Why bother with mapping the damn thing in the caller and having it pinned
> all along whatever ->sendmsg() you end up calling?  Just give it
> page/offset/length instead of address/length and let lib/iov_iter.c
> do the right thing...

Hi Al,

Thanks so much for the time you spent writing this detailed explanation. I=
=20
really appreciated that you spend time for teaching newcomers with kindness=
=20
and patience.

I apologize for this late response, but for the past few days I've been out=
 of=20
city with no PC to email the lists (therefore, the lists would have rejecte=
d=20
my messages from phone with a Gmail app).

Yesterday I saw that Sagi sent an reworked version of my patch according to=
=20
your suggestions. This evening I will send it again as a patch v4 and with =
the=20
cover letter it had in the previous version.

You will be acknowledged with a "Suggested by" tag. Instead Sagi with a "Co-
developed-by" and, of course, with a "Signed-off-by" immediately after the=
=20
previous tag. There will be more information after the three dashes.=20

Again thanks,

=46abio