linux-kernel - Re: [PATCH V3 2/7] mm: move MADV_FREE pages into LRU_INACTIVE

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Date:   Fri, 17 Feb 2017 11:22:02 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Shaohua Li <shli@...com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Kernel-team@...com, mhocko@...e.com, minchan@...nel.org,
        hughd@...gle.com, riel@...hat.com, mgorman@...hsingularity.net,
        akpm@...ux-foundation.org
Subject: Re: [PATCH V3 2/7] mm: move MADV_FREE pages into LRU_INACTIVE_FILE
 list

On Thu, Feb 16, 2017 at 04:35:25PM -0800, Shaohua Li wrote:
> On Thu, Feb 16, 2017 at 12:52:53PM -0500, Johannes Weiner wrote:
> > On Tue, Feb 14, 2017 at 11:36:08AM -0800, Shaohua Li wrote:
> > > @@ -126,4 +126,24 @@ static __always_inline enum lru_list page_lru(struct page *page)
> > >  
> > >  #define lru_to_page(head) (list_entry((head)->prev, struct page, lru))
> > >  
> > > +/*
> > > + * lazyfree pages are clean anonymous pages. They have SwapBacked flag cleared
> > > + * to destinguish normal anonymous pages.
> > > + */
> > > +static inline void set_page_lazyfree(struct page *page)
> > > +{
> > > +	VM_BUG_ON_PAGE(!PageAnon(page) || !PageSwapBacked(page), page);
> > > +	ClearPageSwapBacked(page);
> > > +}
> > > +
> > > +static inline void clear_page_lazyfree(struct page *page)
> > > +{
> > > +	VM_BUG_ON_PAGE(!PageAnon(page) || PageSwapBacked(page), page);
> > > +	SetPageSwapBacked(page);
> > > +}
> > > +
> > > +static inline bool page_is_lazyfree(struct page *page)
> > > +{
> > > +	return PageAnon(page) && !PageSwapBacked(page);
> > > +}
> > 
> > Sorry for not getting to v2 in time, but I have to say I strongly
> > agree with your first iterations and would much prefer this to be
> > open-coded.
> > 
> > IMO this needlessly introduces a new state opaquely called "lazyfree",
> > when really that's just anonymous pages that don't need to be swapped
> > before reclaim - PageAnon && !PageSwapBacked. Very simple MM concept.
> > 
> > That especially shows when we later combine it with page_is_file_cache
> > checks like the next patch does.
> > 
> > The rest of the patch looks good to me.
> 
> Thanks! I do agree checking PageSwapBacked is clearer, but Minchan convinced me
> because of the accounting issue. Where do you suggest we should put the
> accounting to?

I now proposed quite a few changes to the setting and clearing sites,
so it's harder to judge, but AFAICT once those sites are consolidated,
open-coding the stat updates as well shouldn't be too bad, right?

One site to clear during MADV_FREE, one site to set during reclaim.