linux-kernel - Re: [PATCH V4 3/6] mm: move MADV_FREE pages into LRU_INACTIVE

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Message-ID: <20170223182206.GA5686@cmpxchg.org>
Date:   Thu, 23 Feb 2017 13:22:06 -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 V4 3/6] mm: move MADV_FREE pages into LRU_INACTIVE_FILE
 list

On Thu, Feb 23, 2017 at 08:26:03AM -0800, Shaohua Li wrote:
> On Thu, Feb 23, 2017 at 10:58:27AM -0500, Johannes Weiner wrote:
> > Hi Shaohua,
> > 
> > On Wed, Feb 22, 2017 at 10:50:41AM -0800, Shaohua Li wrote:
> > > @@ -268,6 +268,12 @@ static void __activate_page(struct page *page, struct lruvec *lruvec,
> > >  		int lru = page_lru_base_type(page);
> > >  
> > >  		del_page_from_lru_list(page, lruvec, lru);
> > > +		if (PageAnon(page) && !PageSwapBacked(page)) {
> > > +			SetPageSwapBacked(page);
> > > +			/* charge to anon scanned/rotated reclaim_stat */
> > > +			file = 0;
> > > +			lru = LRU_INACTIVE_ANON;
> > > +		}
> > 
> > As per my previous feedback, please remove this. Write-after-free will
> > be caught and handled in the reclaimer, read-after-free is a bug that
> > really doesn't require optimizing page aging for. And we definitely
> > shouldn't declare invalid data suddenly valid because it's being read.
> 
> GUP could run into this. Don't we move the page because it's hot? I think it's
> not just about page aging. If we leave the page there, page reclaim will just
> waste time to reclaim the pages which should't be reclaimed.

There is just no convincing justification to add this code, because it
optimizes something that doesn't have a real world application. If we
just delete this branch, for all intents and purposes the outcome will
be perfectly acceptable.