linux-kernel - Re: [BUG/REGRESSION] THP: broken page count after commit aa88b68c

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Message-ID: <20160603103659.GA23467@node>
Date:	Fri, 3 Jun 2016 13:36:59 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Johannes Weiner <hannes@...xchg.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [BUG/REGRESSION] THP: broken page count after commit aa88b68c

On Thu, Jun 02, 2016 at 12:47:57PM -0700, Hugh Dickins wrote:
> On Thu, 2 Jun 2016, Kirill A. Shutemov wrote:
> > On Thu, Jun 02, 2016 at 05:21:41PM +0200, Gerald Schaefer wrote:
> > > 
> > > The following quick hack fixed the issue:
> > > 
> > > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > > index 0d457e7..c99463a 100644
> > > --- a/mm/swap_state.c
> > > +++ b/mm/swap_state.c
> > > @@ -252,7 +252,10 @@ static inline void free_swap_cache(struct page *page)
> > >  void free_page_and_swap_cache(struct page *page)
> > >  {
> > >  	free_swap_cache(page);
> > > -	put_page(page);
> > > +	if (is_huge_zero_page(page))
> > > +		put_huge_zero_page();
> > > +	else
> > > +		put_page(page);
> > >  }
> > >  
> > >  /*
> > 
> > The fix looks good to me.
> 
> Is there a good reason why the refcount of the huge_zero_page is
> huge_zero_refcount, instead of the refcount of the huge_zero_page?
> Wouldn't the latter avoid such is_huge_zero_page() special-casing?

Hm. I thought I had a reason for not using page's refcount, but I can't
find any now. We would loose sanity check in put_huge_zero_page(), but I
guess it's fine since we never triggered it.

I'll put it to my todo list.

-- 
 Kirill A. Shutemov