Re: [PATCH 7/8] mm: reinstate ZERO_PAGE

[Rik van Riel <riel@redhat.com>]

Hugh Dickins wrote:
> KAMEZAWA Hiroyuki has observed customers of earlier kernels taking
> advantage of the ZERO_PAGE: which we stopped do_anonymous_page() from
> using in 2.6.24.  And there were a couple of regression reports on LKML.
> 
> Following suggestions from Linus, reinstate do_anonymous_page() use of
> the ZERO_PAGE; but this time avoid dirtying its struct page cacheline
> with (map)count updates - let vm_normal_page() regard it as abnormal.
> 
> Use it only on arches which __HAVE_ARCH_PTE_SPECIAL (x86, s390, sh32,
> most powerpc): that's not essential, but minimizes additional branches
> (keeping them in the unlikely pte_special case); and incidentally
> excludes mips (some models of which needed eight colours of ZERO_PAGE
> to avoid costly exceptions).
> 
> Don't be fanatical about avoiding ZERO_PAGE updates: get_user_pages()
> callers won't want to make exceptions for it, so increment its count
> there.  Changes to mlock and migration? happily seems not needed.
> 
> In most places it's quicker to check pfn than struct page address:
> prepare a __read_mostly zero_pfn for that.  Does get_dump_page()
> still need its ZERO_PAGE check? probably not, but keep it anyway.
> 
> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

Acked-by: Rik van Riel <riel@redhat.com>

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