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

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Message-ID: <alpine.DEB.2.11.1508211613130.29152@east.gentwo.org>
Date:	Fri, 21 Aug 2015 16:15:35 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	"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, 21 Aug 2015, Andrew Morton wrote:

> On Fri, 21 Aug 2015 22:31:09 +0300 "Kirill A. Shutemov" <kirill@...temov.name> wrote:
>
> > 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().
>
> That wasn't my question actually...
>
> What I'm wondering is: if this page is being used for slab, will any
> code path ever run PageTail() against it?  If not, we don't need to be
> concerned about that bit.

virt_to_head_page will run PageTail because it uses compound_head(). And
compound_head needs to use the first_page pointer if its a tail page.

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