Re: [RFC][PATCH] getname_maybe_null() - the third variant of pathname copy-in

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Oct 21, 2024 at 02:47:59PM +0200, Christian Brauner wrote:
> On Fri, Oct 18, 2024 at 08:38:22PM +0100, Al Viro wrote:
> > On Fri, Oct 18, 2024 at 05:51:58PM +0100, Al Viro wrote:
> > 
> > > Extra cycles where?  If anything, I'd expect a too-small-to-measure
> > > speedup due to dereference shifted from path_init() to __set_nameidata().
> > > Below is literally all it takes to make filename_lookup() treat NULL
> > > as empty-string name.
> > > 
> > > NOTE: I'm not talking about forcing the pure by-descriptor case through
> > > the dfd+pathname codepath; not without serious profiling.  But treating
> > > AT_FDCWD + NULL by the delta below and passing NULL struct filename to
> > > filename_lookup()?  Where do you expect to have the lost cycles on that?
> > 
> > [snip]
> > 
> > BTW, could you give me a reference to the mail with those objections?
> > I don't see anything in my mailbox, but...
> 
> I had to search for quite a bit myself:
> 
> https://lore.kernel.org/r/CAHk-=wifPKRG2w4mw+YchNtAuk4mMJBde7bG-Z7wt0+ZeQMJ_A@mail.gmail.com

Re get_user() - there's one architecture where this fetch is a clear loss.
Take a look at what um is doing; it's a full page table walk, then
(single-byte) memcpy().  With no caching of page table walk results,
so strncpy_from_user() in case the sucker is _not_ empty will have to
start from scratch (and it's _not_ generic strncpy_from_user() there,
for the same reasons).

BTW, I wonder if we could speed the things up on um by caching the last
page table walk result - and treating that as a TLB.  Might make
back-to-back get_user()/put_user() seriously cheaper there - unsafe_get_user()
could grow a fastpath, possibly making generic strncpy_from_user() cheap
enough to be used.  If that is feasible, the only non-generic variant
would remain on mips, and that's a lot less convincing case than um.
Possibly strnlen_user(), as well - that one has a variant on xtensa,
but that's also not an obvious win compared to generic...

That's a separate story, anyway.