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From:   David Howells <dhowells@redhat.com>
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To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.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()
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Date:   Tue, 25 Apr 2023 09:30:30 +0100
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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 =

> kmap */
> > -		segment->sg_mapped =3D kmap(sg_page(sg));
> > -	}
> > -
> > +	segment->atomic_mapped =3D true;
> > +	segment->sg_mapped =3D kmap_atomic(sg_page(sg));
> =

> As you probably know, kmap_atomic() is deprecated.
> =

> I must admit that I'm not an expert of this code, however, it looks like=
 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 ab=
out =

> the possibility that preemption should be explicitly disabled along with=
 the =

> replacement with kmap_local_page()). =

> =

> Last year I've been working on several conversions from kmap{,_atomic}()=
 to =

> kmap_local_page(), however I'm still not sure to understand what's happe=
ning =

> here...
> =

> Am I missing any important details? Can you please explain why we still =
need =

> that kmap_atomic() instead of kmap_local_page()? =


Actually, it might be worth dropping segment->sg_mapped and segment->data =
and
only doing the kmap_local when necessary.

And this:

			struct msghdr msg =3D { .msg_flags =3D flags };
			struct kvec iov =3D {
				.iov_base =3D segment->data + offset,
				.iov_len =3D copy
			};

			r =3D kernel_sendmsg(sk, &msg, &iov, 1, copy);

should really be using struct bvec, not struct kvec - then the mapping isn=
't
necessary.  It looks like this might be the only place the mapping is used=
,
but I'm not 100% certain.

David