Re: [RFC] mm/memory.c: Optimizing THP zeroing routine for !HIGHMEM cases

[Prathu Baronia <prathu.baronia@oneplus.com>]

The 04/11/2020 13:47, Alexander Duyck wrote:
> 
> This is an interesting data point. So running things in reverse seems
> much more expensive than running them forward. As such I would imagine
> process_huge_page is going to be significantly more expensive then on
> ARM64 since it will wind through the pages in reverse order from the
> end of the page all the way down to wherever the page was accessed.
> 
> I wonder if we couldn't simply process_huge_page to process pages in
> two passes? The first being from the addr_hint + some offset to the
> end, and then loop back around to the start of the page for the second
> pass and just process up to where we started the first pass. The idea
> would be that the offset would be enough so that we have the 4K that
> was accessed plus some range before and after the address hopefully
> still in the L1 cache after we are done.
That's a great idea, we were working on a similar idea for the v2 patch and you
suggesting this idea has reassured our approach. This will incorporate the
benefits of optimized memset and will keep the cache hot around the
faulting address.

Earlier we had taken this offset as 0.5MB and after your response we have kept it
as 32KB. As we understand there is a trade-off associated with keeping this value
too high, we would really appreciate if you can suggest a method to derive an
appropriate value for this offset from the L1 cache size.
> 
> 
> An additional thing I was just wondering is if this also impacts the
> copy operations as well? Looking through the code the two big users
> for process_huge_page are clear_huge_page and copy_user_huge_page. One
> thing that might make more sense than just splitting the code at a
> high level would be to look at possibly refactoring process_huge_page
> and the users for it.
You are right, we didn't consider refactoring process_huge_page earlier. We
will incorporate this in the soon to be sent v2 patch.

Thanks a lot for the interesting insights!

-- 
Prathu Baronia
OnePlus RnD