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Date:   Tue, 3 Mar 2020 17:27:05 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     "Huang, Ying" <ying.huang@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        Zi Yan <ziy@nvidia.com>, Michal Hocko <mhocko@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Minchan Kim <minchan@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH] mm: Add PageLayzyFree() helper functions for MADV_FREE
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References: <20200303033738.281908-1-ying.huang@intel.com> <20200303165859.7440f23d388503ca77fdb6c2@linux-foundation.org>
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On Tue, 3 Mar 2020, Andrew Morton wrote:

> On Tue,  3 Mar 2020 11:37:38 +0800 "Huang, Ying" <ying.huang@intel.com> wrote:
> 
> > From: Huang Ying <ying.huang@intel.com>
> > 
> > Now PageSwapBacked() is used as the helper function to check whether
> > pages have been freed lazily via MADV_FREE.  This isn't very obvious.
> > So Dave suggested to add PageLazyFree() family helper functions to
> > improve the code readability.
> > 
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -498,6 +498,31 @@ static __always_inline int PageKsm(struct page *page)
> >  TESTPAGEFLAG_FALSE(Ksm)
> >  #endif
> >  
> > +/*
> > + * For pages freed lazily via MADV_FREE.  lazyfree pages are clean
> > + * anonymous pages.  They have SwapBacked flag cleared to distinguish
> > + * with normal anonymous pages
> > + */
> > +static __always_inline int PageLazyFree(struct page *page)
> > +{
> > +	page = compound_head(page);
> > +	return PageAnon(page) && !PageSwapBacked(page);
> > +}
> > +
> > +static __always_inline void SetPageLazyFree(struct page *page)
> > +{
> > +	VM_BUG_ON_PAGE(PageTail(page), page);
> > +	VM_BUG_ON_PAGE(!PageAnon(page), page);
> > +	ClearPageSwapBacked(page);
> > +}
> > +
> > +static __always_inline void ClearPageLazyFree(struct page *page)
> > +{
> > +	VM_BUG_ON_PAGE(PageTail(page), page);
> > +	VM_BUG_ON_PAGE(!PageAnon(page), page);
> > +	SetPageSwapBacked(page);
> > +}
> 
> These BUG_ONs aren't present in the current code and are
> unchangelogged.
> 

Yeah, as well as the implicit conversion to check compound_head().