On Fri, 1 Sept 2023 at 08:20, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
>
> cp_new_stat and the counterpart for statx can dodge this rep mov by
> filling user memory directly.
Yeah, they could be made to use the "unsafe_put_user()" machinery
these days, and we could go back to the good old days of avoiding the
extra temp buffer.
> I'm going to patch this, but first I want to address the bigger
> problem of glibc implementing fstat as newfstatat, demolishing perf of
> that op. In their defense currently they have no choice as this is the
> only exporter of the "new" struct stat. I'll be sending a long email
> to fsdevel soon(tm) with a proposed fix.
I wouldn't mind re-instating the "copy directly to user space rather
than go through a temporary buffer", for the stat family of functions,
so please do..
> So I was wondering if rep movsq is any worse than ERMS'ed rep movsb
> when there is no tail to handle and the buffer is aligned to a page,
> or more to the point if clear_page gets any benefit for going with
> movsb.
Hard to tell. 'movsq' is *historically* better, and likely on all
current microarchitectures.
But 'movsb' is actually in many ways easier for the CPU to optimize,
because there's no question of the sub-chunking if anything is not
aligned just rught.
Linus
