Re: [PATCH] mm: make CONFIG_DEFERRED_STRUCT_PAGE_INIT depends on !FLATMEM explicitly

[Shi, Yang]

On 5/23/2016 11:22 AM, Michal Hocko wrote:
> On Mon 23-05-16 09:54:31, Yang Shi wrote:
> > Per the suggestion from Michal Hocko [1], CONFIG_DEFERRED_STRUCT_PAGE_INIT
> > should be incompatible with FLATMEM, make this explicitly in Kconfig.
>
> I guess the changelog could benefit from some clarification. What
> do you think about the following:
>
> "
> DEFERRED_STRUCT_PAGE_INIT requires some ordering wrt other
> initialization operations, e.g. page_ext_init has to happen after the
> whole memmap is initialized properly. For SPARSEMEM this requires to
> wait for page_alloc_init_late. Other memory models (e.g. flatmem) might
> have different initialization layouts (page_ext_init_flatmem). Currently
> DEFERRED_STRUCT_PAGE_INIT depends on MEMORY_HOTPLUG which in turn
> 	depends on SPARSEMEM || X86_64_ACPI_NUMA
> 	depends on ARCH_ENABLE_MEMORY_HOTPLUG
>
> and X86_64_ACPI_NUMA depends on NUMA which in turn disable FLATMEM
> memory model:
> config ARCH_FLATMEM_ENABLE
> 	def_bool y
> 	depends on X86_32 && !NUMA
>
> so FLATMEM is ruled out via dependency maze. Be explicit and disable
> FLATMEM for DEFERRED_STRUCT_PAGE_INIT so that we do not reintroduce
> subtle initialization bugs
> "

Thanks a lot. It sounds way better. Will address in V2.

Yang

> > [1] http://lkml.kernel.org/r/20160523073157.GD2278@xxxxxxxxxxxxxx
> >
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>
> > ---
> >  mm/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 2664c11..22fa818 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -649,6 +649,7 @@ config DEFERRED_STRUCT_PAGE_INIT
> >  	default n
> >  	depends on ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
> >  	depends on MEMORY_HOTPLUG
> > +	depends on !FLATMEM
> >  	help
> >  	  Ordinarily all struct pages are initialised during early boot in a
> >  	  single thread. On very large machines this can take a considerable
> > --
> > 2.0.2