Re: [PATCHv3 4/5] mm: make compound_head() robust

[Vlastimil Babka]

On 08/21/2015 09:34 PM, Andrew Morton wrote:
> On Fri, 21 Aug 2015 22:31:09 +0300 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> 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.

Pfn scanners such as compaction might inspect such pages and run 
compound_head() (and thus PageTail) on them. I think no kind of page 
within a zone (slab or otherwise) is "protected" from this, which is why 
it needs to be robust.

> And slab was just the example I chose.  The same question petains to
> all other uses of that union.

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