On Mon, 22 May 2023 10:42:12 -0700 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > 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. 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
