linux-kernel - Re: [PATCH v3 4/9] mm/swapcache: support to handle the value in swapcache

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Date:   Wed, 18 Mar 2020 14:33:18 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     js1304@...il.com
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>, kernel-team@....com,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v3 4/9] mm/swapcache: support to handle the value in
 swapcache

On Tue, Mar 17, 2020 at 02:41:52PM +0900, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> Swapcache doesn't handle the value since there is no case using the value.
> In the following patch, workingset detection for anonymous page will be
> implemented and it stores the value into the swapcache. So, we need to
> handle it and this patch implement handling.

"value" is too generic, it's not quite clear what this refers to
here. "Exceptional entries" or "shadow entries" would be better.

> @@ -155,24 +163,33 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp)
>   * This must be called only on pages that have
>   * been verified to be in the swap cache.
>   */
> -void __delete_from_swap_cache(struct page *page, swp_entry_t entry)
> +void __delete_from_swap_cache(struct page *page,
> +			swp_entry_t entry, void *shadow)
>  {
>  	struct address_space *address_space = swap_address_space(entry);
>  	int i, nr = hpage_nr_pages(page);
>  	pgoff_t idx = swp_offset(entry);
>  	XA_STATE(xas, &address_space->i_pages, idx);
>  
> +	/* Do not apply workingset detection for the hugh page */
> +	if (nr > 1)
> +		shadow = NULL;

Hm, why is that? Should that be an XXX/TODO item? The comment should
explain the reason, not necessarily what the code is doing.

Also, s/hugh/huge/

The rest of the patch looks straight-forward to me.