On Fri, 21 Aug 2015, Andrew Morton wrote:

On Fri, 21 Aug 2015 22:31:09 +0300 "Kirill A. Shutemov" [off-list ref] wrote:

quoted

On Fri, Aug 21, 2015 at 11:11:27AM -0500, Christoph Lameter wrote:

quoted

On Fri, 21 Aug 2015, Kirill A. Shutemov wrote:

quoted

quoted

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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