Re: [PATCH] HitShield:Something new eviction process for MGLRU

[Matthew Wilcox <willy@infradead.org>]

On Sat, Jul 20, 2024 at 11:25:28PM +0900, Minwoo Jo wrote:
> This hitshield technique was devised based on the observation that MGLRU
> does not consider the state of the folio when performing eviction.
> 
> The assumption is that if a folio that has been updated 1-3 times and not
> frequently updated accumulates generations until it is evicted, it is
> likely to have a low probability of being referenced in the future.

Hi Minwoo.  This is interesting work.  Thank you for sending it out.
I do not consider myself qualified to comment on your assumption, but
I have some feedback on your implementation.

> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index a199c48bc462..053d5620574e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -189,6 +189,12 @@ struct page {
>  	void *virtual;			/* Kernel virtual address (NULL if
>  					   not kmapped, ie. highmem) */
>  #endif /* WANT_PAGE_VIRTUAL */
> +	unsigned char hit_shield;	/*
> +					 * The hit_shield variable I added to the page
> +					 * This variable is responsible for counting
> +					 * the number of times this
> +					 * page's generation has been updated.
> +					 */

We don't really have space in struct page for this kind of thing,
unfortunately.  There are a _few_ bits available in page->flags on
64-bit, but even that is a bit tenuous.