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;
MIME-Version: 1.0
References: <20230519074047.1739879-1-dhowells@redhat.com> <20230519074047.1739879-24-dhowells@redhat.com>
 <20230522102920.0528d821@rorschach.local.home> <2812412.1684767005@warthog.procyon.org.uk>
In-Reply-To: <2812412.1684767005@warthog.procyon.org.uk>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 22 May 2023 10:42:12 -0700
Message-ID: <CAHk-=wgg4iDEuSN4K6S6ohAm4zd_V5h4tXGn6-2-cfOuJPFDZQ@mail.gmail.com>
Subject: Re: [PATCH v20 23/32] splice: Convert trace/seq to use direct_splice_read()
To:     David Howells <dhowells@redhat.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>, Jens Axboe <axboe@kernel.dk>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Hillf Danton <hdanton@sina.com>,
        Christian Brauner <brauner@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Christoph Hellwig <hch@lst.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        linux-trace-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Mon, May 22, 2023 at 7:50=E2=80=AFAM David Howells <dhowells@redhat.com>=
 wrote:
>
> We could implement seq_splice_read().  What we would need to do is to cha=
nge
> how the seq buffer is allocated: bulk allocate a bunch of arbitrary pages
> which we then vmap().  When we need to splice, we read into the buffer, d=
o a
> vunmap() and then splice the pages holding the data we used into the pipe=
.

Please don't use vmap as a way to do zero-copy.

The virtual mapping games are more expensive than a small copy from
some random seq file.

Yes, yes, seq_file currently uses "kvmalloc()", which does fall back
to vmalloc too. But the keyword there is "falls back". Most of the
time it's just a regular boring kmalloc, and most of the time a
seq-file is tiny.

                      Linus