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Date:   Mon, 22 May 2023 14:38:14 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Howells <dhowells@redhat.com>, 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>,
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        Christoph Hellwig <hch@lst.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH v20 23/32] splice: Convert trace/seq to use
 direct_splice_read()
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On Mon, 22 May 2023 10:42:12 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, May 22, 2023 at 7:50 AM David Howells <dhowells@redhat.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