On Mon, 22 May 2023 10:42:12 -0700
Linus Torvalds [off-list ref] wrote:

On Mon, May 22, 2023 at 7:50 AM David Howells [off-list ref] wrote:

quoted

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