linux-kernel - Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

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Date:   Wed, 9 Aug 2023 20:21:16 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

On Wed, Aug 09, 2023 at 08:07:43PM +0100, Ryan Roberts wrote:
> > +++ b/mm/hugetlb.c
> > @@ -1479,7 +1479,7 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
> >  	struct page *p;
> >  
> >  	atomic_set(&folio->_entire_mapcount, 0);
> > -	atomic_set(&folio->_nr_pages_mapped, 0);
> > +	atomic_set(&folio->_total_mapcount, 0);
> 
> Just checking this is definitely what you intended? _total_mapcount is -1 when
> it means "no pages mapped", so 0 means 1 page mapped?

We're destroying the page here, so rather than setting the meaning of
this, we're setting the contents of this memory to 0.

Other thoughts that ran through my mind ... can we wrap?  I don't think
we can; we always increment total_mapcount by 1, no matter whether we're
incrementing entire_mapcount or an individual page's mapcount, and we
always call folio_get() first, so we can't increment total_mapcount
past 2^32 because folio_get() will die first.  We might be able to
wrap past 2^31, but I don't think so.

I had some other thoughts, but I convinced myself they were all OK.