Re: [PATCH v3 00/21] huge page clearing optimizations

[Ankur Arora]

[ Fixed email for Joao Martins. ]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Mon, Jun 6, 2022 at 1:22 PM Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:
[snip]

> So I really don't like this series. A *lot* of it comes from that
> horrible process_huge_page() model, and the whole model is just wrong
> and pointless. You're literally trying to fix the mess that that
> function is, but you're keeping the fundamental problem around.
>
> The whole *point* of your patch-set is to use non-temporal stores,
> which makes all the process_huge_page() things entirely pointless, and
> only complicates things.
>
> And even if we don't use non-temporal stores, that process_huge_page()
> thing makes for trouble for any "rep stos/movs" implementation that
> might actualyl do a better job if it was just chunked up in bigger
> chunks.

This makes sense to me. There is a lot of unnecessary machinery
around process_huge_page() and this series adds more of it.

For highmem and page-at-a-time archs we would need to keep some
of the same optimizations (via the common clear/copy_user_highpages().)

Still that rids the arch code from pointless constraints as you
say below.

> Yes, yes, you probably still want to chunk that up somewhat due to
> latency reasons, but even then architectures might as well just make
> their own decisions, rather than have the core mm code make one
> clearly bad decision for them. Maybe chunking it up in bigger chunks
> than one page.

Right. Or doing the whole contiguous area in one or a few chunks
chunks, and then touching the faulting cachelines towards the end.

> Maybe an architecture could do even more radical things like "let's
> just 'rep stos' for the whole area, but set a special thread flag that
> causes the interrupt return to break it up on return to kernel space".
> IOW, the "latency fix" might not even be about chunking it up, it
> might look more like our exception handling thing.

When I was thinking about this earlier, I had a vague inkling of
setting a thread flag and defer writes to the last few cachelines
for just before returning to user-space.
Can you elaborate a little about what you are describing above?

> So I really think that crapectomy should be the first thing you do,
> and that should be that first part of "extends the clear_page()
> machinery to also handle extents larger than a single page"

Ack that. And, thanks for the detailed comments.

--
ankur