netdev - Re: [PATCH v4 4/4] libceph: use sendpages_ok() instead of sendpage

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Message-ID: <CAOi1vP96JtsP02hpsZpeknKN_dh3JdnQomO8aTbuH6Bz247rxA@mail.gmail.com>
Date: Wed, 17 Jul 2024 22:26:10 +0200
From: Ilya Dryomov <idryomov@...il.com>
To: Ofir Gal <ofir.gal@...umez.com>
Cc: davem@...emloft.net, linux-block@...r.kernel.org, 
	linux-nvme@...ts.infradead.org, netdev@...r.kernel.org, 
	ceph-devel@...r.kernel.org, dhowells@...hat.com, edumazet@...gle.com, 
	pabeni@...hat.com, kbusch@...nel.org, xiubli@...hat.com
Subject: Re: [PATCH v4 4/4] libceph: use sendpages_ok() instead of sendpage_ok()

On Tue, Jul 16, 2024 at 2:46 PM Ofir Gal <ofir.gal@...umez.com> wrote:
>
> Xiubo/Ilya please take a look
>
> On 6/11/24 09:36, Ofir Gal wrote:
> > Currently ceph_tcp_sendpage() and do_try_sendpage() use sendpage_ok() in
> > order to enable MSG_SPLICE_PAGES, it check the first page of the
> > iterator, the iterator may represent contiguous pages.
> >
> > MSG_SPLICE_PAGES enables skb_splice_from_iter() which checks all the
> > pages it sends with sendpage_ok().
> >
> > When ceph_tcp_sendpage() or do_try_sendpage() send an iterator that the
> > first page is sendable, but one of the other pages isn't
> > skb_splice_from_iter() warns and aborts the data transfer.
> >
> > Using the new helper sendpages_ok() in order to enable MSG_SPLICE_PAGES
> > solves the issue.
> >
> > Signed-off-by: Ofir Gal <ofir.gal@...umez.com>
> > ---
> >  net/ceph/messenger_v1.c | 2 +-
> >  net/ceph/messenger_v2.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/ceph/messenger_v1.c b/net/ceph/messenger_v1.c
> > index 0cb61c76b9b8..a6788f284cd7 100644
> > --- a/net/ceph/messenger_v1.c
> > +++ b/net/ceph/messenger_v1.c
> > @@ -94,7 +94,7 @@ static int ceph_tcp_sendpage(struct socket *sock, struct page *page,
> >        * coalescing neighboring slab objects into a single frag which
> >        * triggers one of hardened usercopy checks.
> >        */
> > -     if (sendpage_ok(page))
> > +     if (sendpages_ok(page, size, offset))
> >               msg.msg_flags |= MSG_SPLICE_PAGES;
> >
> >       bvec_set_page(&bvec, page, size, offset);
> > diff --git a/net/ceph/messenger_v2.c b/net/ceph/messenger_v2.c
> > index bd608ffa0627..27f8f6c8eb60 100644
> > --- a/net/ceph/messenger_v2.c
> > +++ b/net/ceph/messenger_v2.c
> > @@ -165,7 +165,7 @@ static int do_try_sendpage(struct socket *sock, struct iov_iter *it)
> >                * coalescing neighboring slab objects into a single frag
> >                * which triggers one of hardened usercopy checks.
> >                */
> > -             if (sendpage_ok(bv.bv_page))
> > +             if (sendpages_ok(bv.bv_page, bv.bv_len, bv.bv_offset))
> >                       msg.msg_flags |= MSG_SPLICE_PAGES;
> >               else
> >                       msg.msg_flags &= ~MSG_SPLICE_PAGES;
>

Hi Ofir,

Ceph should be fine as is -- there is an internal "cursor" abstraction
that that is limited to PAGE_SIZE chunks, using bvec_iter_bvec() instead
of mp_bvec_iter_bvec(), etc.  This means that both do_try_sendpage() and
ceph_tcp_sendpage() should be called only with

  page_off + len <= PAGE_SIZE

being true even if the page is contiguous (and that we lose out on the
potential performance benefit, of course...).

That said, if the plan is to remove sendpage_ok() so that it doesn't
accidentally grow new users who are unaware of this pitfall, consider
this

Acked-by: Ilya Dryomov <idryomov@...il.com>

Thanks,

                Ilya