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

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Message-ID: <20230522143814.6efbbb4d@gandalf.local.home>
Date:   Mon, 22 May 2023 14:38:14 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>, 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, 22 May 2023 10:42:12 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 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.

I was thinking this change had to do with the splice callback for
trace_pipe_raw (which is a hot path that does zero copy of the ftrace ring
buffer into files). But looking at this further, I see that it's for just
the "trace" file, which is a textual conversion of the tracing data (slow
path, although some user space uses this and parses the text, which IMHO is
wrong).

In other words, I don't really care much about this code being "efficient".

-- Steve