linux-kernel - Re: [PATCH v20 23/32] splice: Convert trace/seq to use direct_splice

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Date:   Mon, 22 May 2023 10:42:12 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@....de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v20 23/32] splice: Convert trace/seq to use direct_splice_read()

On Mon, May 22, 2023 at 7:50 AM David Howells <dhowells@...hat.com> wrote:
>
> We could implement seq_splice_read().  What we would need to do is to change
> 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, do 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