Re: [PATCH v1 0/9] mm/memory: optimize unmap/zap with PTE-mapped THP

Yin Fengwei
2024-01-31T02:20:44Z

On 1/29/24 22:32, David Hildenbrand wrote:
> This series is based on [1] and must be applied on top of it.
> Similar to what we did with fork(), let's implement PTE batching
> during unmap/zap when processing PTE-mapped THPs.
>
> We collect consecutive PTEs that map consecutive pages of the same large
> folio, making sure that the other PTE bits are compatible, and (a) adjust
> the refcount only once per batch, (b) call rmap handling functions only
> once per batch, (c) perform batch PTE setting/updates and (d) perform TLB
> entry removal once per batch.
>
> Ryan was previously working on this in the context of cont-pte for
> arm64, int latest iteration [2] with a focus on arm6 with cont-pte only.
> This series implements the optimization for all architectures, independent
> of such PTE bits, teaches MMU gather/TLB code to be fully aware of such
> large-folio-pages batches as well, and amkes use of our new rmap batching
> function when removing the rmap.
>
> To achieve that, we have to enlighten MMU gather / page freeing code
> (i.e., everything that consumes encoded_page) to process unmapping
> of consecutive pages that all belong to the same large folio. I'm being
> very careful to not degrade order-0 performance, and it looks like I
> managed to achieve that.

One possible scenario:
If all the folio is 2M size folio, then one full batch could hold 510M memory.
Is it too much regarding one full batch before just can hold (2M - 4096 * 2)
memory?

Regards
Yin, Fengwei