linux-kernel - Re: [PATCHv3 4/5] mm: make compound

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Message-ID: <20150821193109.GA14785@node.dhcp.inet.fi>
Date:	Fri, 21 Aug 2015 22:31:09 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust

On Fri, Aug 21, 2015 at 11:11:27AM -0500, Christoph Lameter wrote:
> On Fri, 21 Aug 2015, Kirill A. Shutemov wrote:
> 
> > > Is this really true?  For example if it's a slab page, will that page
> > > ever be inspected by code which is looking for the PageTail bit?
> >
> > +Christoph.
> >
> > What we know for sure is that space is not used in tail pages, otherwise
> > it would collide with current compound_dtor.
> 
> Sl*b allocators only do a virt_to_head_page on tail pages.

The question was whether it's safe to assume that the bit 0 is always zero
in the word as this bit will encode PageTail().

-- 
 Kirill A. Shutemov
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