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:     David Howells <dhowells@redhat.com>
Cc:     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>,
        David Howells <dhowells@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:   Mon, 24 Apr 2023 19:19:04 +0200
Message-ID: <1957131.PYKUYFuaPT@suse>
In-Reply-To: <20230331160914.1608208-42-dhowells@redhat.com>
References: <20230331160914.1608208-1-dhowells@redhat.com>
 <20230331160914.1608208-42-dhowells@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="iso-8859-1"
Precedence: bulk

On venerd=EC 31 marzo 2023 18:09:00 CEST David Howells wrote:
> As iscsi is now using sendmsg() with MSG_SPLICE_PAGES rather than sendpag=
e,
> assume that sendpage_ok() will return true in iscsi_tcp_segment_map() and
> leave it to TCP to copy the data if not.
>=20
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: "Martin K. Petersen" <martin.petersen@oracle.com>
> cc: "David S. Miller" <davem@davemloft.net>
> cc: Eric Dumazet <edumazet@google.com>
> cc: Jakub Kicinski <kuba@kernel.org>
> cc: Paolo Abeni <pabeni@redhat.com>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: linux-scsi@vger.kernel.org
> cc: target-devel@vger.kernel.org
> cc: netdev@vger.kernel.org
> ---
>  drivers/scsi/libiscsi_tcp.c | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
>=20
> diff --git a/drivers/scsi/libiscsi_tcp.c b/drivers/scsi/libiscsi_tcp.c
> index c182aa83f2c9..07ba0d864820 100644
> --- a/drivers/scsi/libiscsi_tcp.c
> +++ b/drivers/scsi/libiscsi_tcp.c
> @@ -128,18 +128,11 @@ static void iscsi_tcp_segment_map(struct iscsi_segm=
ent
> *segment, int recv) * coalescing neighboring slab objects into a single f=
rag
> which
>  	 * triggers one of hardened usercopy checks.
>  	 */
> -	if (!recv && sendpage_ok(sg_page(sg)))
> +	if (!recv)
>  		return;
>=20
> -	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 */
> -		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 th=
e=20
mapping has no need to rely on the side effects of kmap_atomic() (i.e.,=20
pagefault_disable() and preempt_disable() - but I'm not entirely sure about=
=20
the possibility that preemption should be explicitly disabled along with th=
e=20
replacement with kmap_local_page()).=20

Last year I've been working on several conversions from kmap{,_atomic}() to=
=20
kmap_local_page(), however I'm still not sure to understand what's happenin=
g=20
here...

Am I missing any important details? Can you please explain why we still nee=
d=20
that kmap_atomic() instead of kmap_local_page()?=20

Thanks in advance,

=46abio

>  	segment->data =3D segment->sg_mapped + sg->offset + segment-
>sg_offset;
>  }