Re: [PATCH v3 41/55] iscsi: Assume "sendpage" is okay in iscsi_tcp_segment_map()

Fabio M. De Francesco
2023-04-24T17:19:24Z

On venerdì 31 marzo 2023 18:09:00 CEST David Howells wrote:
> As iscsi is now using sendmsg() with MSG_SPLICE_PAGES rather than sendpage,
> assume that sendpage_ok() will return true in iscsi_tcp_segment_map() and
> leave it to TCP to copy the data if not.
>
> 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(-)
>
> 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_segment
> *segment, int recv) * coalescing neighboring slab objects into a single frag
> which
> * triggers one of hardened usercopy checks.
> */
> - if (!recv && sendpage_ok(sg_page(sg)))
> + if (!recv)
> return;
>
> - if (recv) {
> - segment->atomic_mapped = true;
> - segment->sg_mapped = kmap_atomic(sg_page(sg));
> - } else {
> - segment->atomic_mapped = false;
> - /* the xmit path can sleep with the page mapped so use
kmap */
> - segment->sg_mapped = kmap(sg_page(sg));
> - }
> -
> + segment->atomic_mapped = true;
> + segment->sg_mapped = 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 about
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 happening
here...

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

Thanks in advance,

Fabio

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