Re: [PATCH v5 5/9] mm: swap: skip slot cache on freeing for mTHP

[Barry Song]

On Wed, Jul 31, 2024 at 6:49 PM <chrisl@xxxxxxxxxx> wrote:
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> Currently when we are freeing mTHP folios from swap cache, we free
> then one by one and put each entry into swap slot cache. Slot
> cache is designed to reduce the overhead by batching the freeing,
> but mTHP swap entries are already continuous so they can be batch
> freed without it already, it saves litle overhead, or even increase
> overhead for larger mTHP.
>
> What's more, mTHP entries could stay in swap cache for a while.
> Contiguous swap entry is an rather rare resource so releasing them
> directly can help improve mTHP allocation success rate when under
> pressure.
>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>

Acked-by: Barry Song <baohua@xxxxxxxxxx>

I believe this is the right direction to take. Currently, entries are released
one by one, even when they are contiguous in the swap file(those nr_pages
entries are definitely in the same cluster and same si), leading to numerous
lock and unlock operations.
This approach provides batched support.

free_swap_and_cache_nr() has the same issue, so I drafted a patch based on
your code. I wonder if you can also help test and review before I send it
officially: