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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     David Howells <dhowells@redhat.com>
Cc:     dhowells@redhat.com, Matthew Wilcox <willy@infradead.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jeff Layton <jlayton@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Chuck Lever III <chuck.lever@oracle.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        netdev@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        linux-scsi@vger.kernel.org, target-devel@vger.kernel.org
Subject: Re: [PATCH v3 41/55] iscsi: Assume "sendpage" is okay in
 iscsi_tcp_segment_map()
Date:   Tue, 25 Apr 2023 15:13:33 +0200
Message-ID: <16526634.geO5KgaWL5@suse>
In-Reply-To: <494037.1682411430@warthog.procyon.org.uk>
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 <494037.1682411430@warthog.procyon.org.uk>
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On marted=EC 25 aprile 2023 10:30:30 CEST David Howells wrote:
> Fabio M. De Francesco <fmdefrancesco@gmail.com> wrote:
> > > -	if (recv) {
> > > -		segment->atomic_mapped =3D true;
> > > -		segment->sg_mapped =3D kmap_atomic(sg_page(sg));
> > > -	} else {
> > > -		segment->atomic_mapped =3D false;
> > > -		/* the xmit path can sleep with the page mapped so use
> >=20
> > kmap */
> >=20
> > > -		segment->sg_mapped =3D kmap(sg_page(sg));
> > > -	}
> > > -
> > > +	segment->atomic_mapped =3D true;
> > > +	segment->sg_mapped =3D kmap_atomic(sg_page(sg));
> >=20
> > As you probably know, kmap_atomic() is deprecated.
> >=20
> > I must admit that I'm not an expert of this code, however, it looks lik=
e=20
the
> > mapping has no need to rely on the side effects of kmap_atomic() (i.e.,
> > pagefault_disable() and preempt_disable() - but I'm not entirely sure=20
about
> > the possibility that preemption should be explicitly disabled along wit=
h=20
the
> > replacement with kmap_local_page()).
> >=20
> > Last year I've been working on several conversions from kmap{,_atomic}(=
)=20
to
> > kmap_local_page(), however I'm still not sure to understand what's=20
happening
> > here...
> >=20
> > Am I missing any important details? Can you please explain why we still=
=20
need
> > that kmap_atomic() instead of kmap_local_page()?
>=20
> Actually, it might be worth dropping segment->sg_mapped and segment->data=
=20
and
> only doing the kmap_local when necessary.
>=20
> And this:
>=20
> 			struct msghdr msg =3D { .msg_flags =3D flags };
> 			struct kvec iov =3D {
> 				.iov_base =3D segment->data + offset,
> 				.iov_len =3D copy
> 			};
>=20
> 			r =3D kernel_sendmsg(sk, &msg, &iov, 1, copy);
>=20
> should really be using struct bvec, not struct kvec - then the mapping is=
n't
> necessary.

=46WIW, struct bvec looks better suited (despite I have very little knowled=
ge of=20
this code).

I assume that you noticed that we also have the unmapping counterpart=20
(iscsi_tcp_segment_unmap()) which should also be addressed accordingly.

> It looks like this might be the only place the mapping is used,
> but I'm not 100% certain.

It seems that kmap_atomic() (as well as kmap(), which you deleted) is only=
=20
called by iscsi_tcp_segment_map(), which in turn is called only by =20
iscsi_tcp_segment_done(). I can't see any other places where the mapping is=
=20
used.

I hope that this dialogue may help you somehow to choose the best suited wa=
y=20
to get rid of that deprecated kmap_atomic().

Thanks for taking time to address questions from newcomers :-)=20

=46abio

>=20
> David