linux-kernel - Re: [PATCH V2 3/7] mm: reclaim MADV

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Date:   Mon, 13 Feb 2017 14:06:36 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Shaohua Li <shli@...com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Kernel-team@...com, danielmicay@...il.com, mhocko@...e.com,
        hughd@...gle.com, hannes@...xchg.org, riel@...hat.com,
        mgorman@...hsingularity.net, akpm@...ux-foundation.org
Subject: Re: [PATCH V2 3/7] mm: reclaim MADV_FREE pages

On Fri, Feb 10, 2017 at 09:43:07AM -0800, Shaohua Li wrote:

< snip >

> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 947ab6f..b304a84 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -864,7 +864,7 @@ static enum page_references page_check_references(struct page *page,
> > >  		return PAGEREF_RECLAIM;
> > >  
> > >  	if (referenced_ptes) {
> > > -		if (PageSwapBacked(page))
> > > +		if (PageSwapBacked(page) || PageAnon(page))
> > 
> > If anyone accesses MADV_FREEed range with load op, not store,
> > why shouldn't we discard that pages?
> 
> Don't have strong opinion about this, userspace probably shouldn't do this. I'm
> ok to delete it if you insist.

Yes, I prefer to removing unnecessary code unless there is a some reaason.

> 
> > >  			return PAGEREF_ACTIVATE;
> > >  		/*
> > >  		 * All mapped pages start out with page table

< snip >

> > > @@ -971,7 +971,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > >  		int may_enter_fs;
> > >  		enum page_references references = PAGEREF_RECLAIM_CLEAN;
> > >  		bool dirty, writeback;
> > > -		bool lazyfree = false;
> > > +		bool lazyfree;
> > >  		int ret = SWAP_SUCCESS;
> > >  
> > >  		cond_resched();
> > > @@ -986,6 +986,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > >  
> > >  		sc->nr_scanned++;
> > >  
> > > +		lazyfree = page_is_lazyfree(page);
> > > +
> > >  		if (unlikely(!page_evictable(page)))
> > >  			goto cull_mlocked;
> > >  
> > > @@ -993,7 +995,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > >  			goto keep_locked;
> > >  
> > >  		/* Double the slab pressure for mapped and swapcache pages */
> > > -		if (page_mapped(page) || PageSwapCache(page))
> > > +		if ((page_mapped(page) || PageSwapCache(page)) && !lazyfree)
> > >  			sc->nr_scanned++;
> > 
> > In this phase, we cannot know whether lazyfree marked page is discarable
> > or not. If it is freeable and mapped, this logic makes sense. However,
> > if the page is dirty?
> 
> I think this doesn't matter. If the page is dirty, it will go to reclaim in
> next round and swap out. At that time, we will add nr_scanned there.

If the lazyfree page in LRU comes around again into this, it's true but
the page could be freed before that.
Having said that, I don't know how critical it is and what kinds of rationale
was to push slab reclaim so I don't insist on it.

Thanks.