Re: [PATCH v20 23/32] splice: Convert trace/seq to use direct_splice_read()

[Linus Torvalds]

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