linux-kernel - Re: [PATCH v6 2/6] mm/vmscan: protect the workingset on anonymous LRU

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Date:   Fri, 3 Jul 2020 09:47:47 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>, kernel-team@....com,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v6 2/6] mm/vmscan: protect the workingset on anonymous LRU

2020년 7월 2일 (목) 오전 3:02, Vlastimil Babka <vbabka@...e.cz>님이 작성:
>
> On 6/17/20 7:26 AM, js1304@...il.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> Hi, how about a more descriptive subject, such as

Hello,

> mm/vmscan: add new anonymous pages to inactive LRU list

This patch does two things to implement workingset protection.

- add new anonymous pages to inactive LRU list
- check two reference to activate

So, I think that the current subject is more descriptive for this patch.

> > 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.
>
> detection similar to the one that's already applied to file LRU.

Will change!

> > v6: Before this patch, all anon pages (inactive + active) are considered
> > as workingset. However, with this patch, only active pages are considered
> > as workingset. So, file refault formula which uses the number of all
> > anon pages is changed to use only the number of active anon pages.
>
> a "v6" note is more suitable for a diffstat area than commit log, but it's good
> to mention this so drop the 'v6:'?

Okay.

> > Acked-by: Johannes Weiner <hannes@...xchg.org>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

> One more nit below.
>
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -476,23 +476,24 @@ void lru_cache_add(struct page *page)
> >  EXPORT_SYMBOL(lru_cache_add);
> >
> >  /**
> > - * lru_cache_add_active_or_unevictable
> > + * lru_cache_add_inactive_or_unevictable
> >   * @page:  the page to be added to LRU
> >   * @vma:   vma in which page is mapped for determining reclaimability
> >   *
> > - * Place @page on the active or unevictable LRU list, depending on its
> > + * Place @page on the inactive or unevictable LRU list, depending on its
> >   * evictability.  Note that if the page is not evictable, it goes
> >   * directly back onto it's zone's unevictable list, it does NOT use a
> >   * per cpu pagevec.
> >   */
> > -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 unevictable;
> > +
> >       VM_BUG_ON_PAGE(PageLRU(page), page);
> >
> > -     if (likely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) != VM_LOCKED))
> > -             SetPageActive(page);
> > -     else if (!TestSetPageMlocked(page)) {
> > +     unevictable = (vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) == VM_LOCKED;
> > +     if (unevictable && !TestSetPageMlocked(page)) {
>
> I guess this could be "if (unlikely(unevictable) && ..." to match the previous
> if (likely(evictable)) else ...

I will fix it, too.

Thanks.