Re: Optimizing small reads

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 7 Oct 2025 at 15:54, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So here's the slightly fixed patch that actually does boot - and that
> I'm running right now. But I wouldn't call it exactly "tested".
>
> Caveat patchor.

From a quick look at profiles, the major issue is that clac/stac is
very expensive on the machine I'm testing this on, and that makes the
looping over smaller copies unnecessarily costly.

And the iov_iter overhead is quite costly too.

Both would be fixed by instead of just checking the iov_iter_count(),
we should likely check just the first iov_iter entry, and make sure
it's a user space iterator.

Then we'd be able to use the usual - and *much* cheaper -
user_access_begin/end() and unsafe_copy_to_user() functions, and do
the iter update at the end outside the loop.

Anyway, this all feels fairly easily fixable and not some difficult
fundamental issue, but it just requires being careful and getting the
small details right. Not difficult, just "care needed".

But even without that, and in this simplistic form, this should
*scale* beautifully, because all the overheads are purely CPU-local.
So it does avoid the whole atomic page reference stuff etc
synchronization.

                  Linus