linux-kernel - Re: [PATCH v3 2/9] mm/vmscan: protect the workingset on anonymous LRU

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Message-ID: <20200318175155.GB154135@cmpxchg.org>
Date:   Wed, 18 Mar 2020 13:51:55 -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 2/9] mm/vmscan: protect the workingset on anonymous LRU

On Tue, Mar 17, 2020 at 02:41:50PM +0900, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> In current implementation, newly created or swap-in anonymous page
> is started on active list. Growing active list results in rebalancing
> active/inactive list so old pages on active list are demoted to inactive
> list. Hence, the page on active list isn't protected at all.
> 
> Following is an example of this situation.
> 
> Assume that 50 hot pages on active list. Numbers denote the number of
> pages on active/inactive list (active | inactive).
> 
> 1. 50 hot pages on active list
> 50(h) | 0
> 
> 2. workload: 50 newly created (used-once) pages
> 50(uo) | 50(h)
> 
> 3. workload: another 50 newly created (used-once) pages
> 50(uo) | 50(uo), swap-out 50(h)
> 
> This patch tries to fix this issue.
> Like as file LRU, newly created or swap-in anonymous pages will be
> inserted to the inactive list. They are promoted to active list if
> enough reference happens. This simple modification changes the above
> example as following.
> 
> 1. 50 hot pages on active list
> 50(h) | 0
> 
> 2. workload: 50 newly created (used-once) pages
> 50(h) | 50(uo)
> 
> 3. workload: another 50 newly created (used-once) pages
> 50(h) | 50(uo), swap-out 50(uo)
> 
> As you can see, hot pages on active list would be protected.
> 
> Note that, this implementation has a drawback that the page cannot
> be promoted and will be swapped-out if re-access interval is greater than
> the size of inactive list but less than the size of total(active+inactive).
> To solve this potential issue, following patch will apply workingset
> detection that is applied to file LRU some day before.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

Acked-by: Johannes Weiner <hannes@...xchg.org>

> -void lru_cache_add_active_or_unevictable(struct page *page,
> +void lru_cache_add_inactive_or_unevictable(struct page *page,
>  					 struct vm_area_struct *vma)
>  {
> +	bool evictable;
> +
>  	VM_BUG_ON_PAGE(PageLRU(page), page);
>  
> -	if (likely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) != VM_LOCKED))
> -		SetPageActive(page);
> -	else if (!TestSetPageMlocked(page)) {
> +	evictable = (vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) != VM_LOCKED;
> +	if (!evictable && !TestSetPageMlocked(page)) {

Minor point, but in case there is a v4: `unevictable` instead of
!evictable would be a bit easier to read, match the function name,
PageUnevictable etc.