Re: [PATCH] mm: sparse: Skip no-map regions in memblocks_present

["Raslan, KarimAllah" <karahmed@amazon.de>]

On Fri, 2019-07-12 at 23:09 +0000, Wei Yang wrote:
> On Fri, Jul 12, 2019 at 10:51:31AM +0200, KarimAllah Ahmed wrote:
> > 
> > Do not mark regions that are marked with nomap to be present, otherwise
> > these memblock cause unnecessarily allocation of metadata.
> > 
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
> > Cc: Oscar Salvador <osalvador@suse.de>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Mike Rapoport <rppt@linux.ibm.com>
> > Cc: Baoquan He <bhe@redhat.com>
> > Cc: Qian Cai <cai@lca.pw>
> > Cc: Wei Yang <richard.weiyang@gmail.com>
> > Cc: Logan Gunthorpe <logang@deltatee.com>
> > Cc: linux-mm@kvack.org
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> > ---
> > mm/sparse.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> > 
> > diff --git a/mm/sparse.c b/mm/sparse.c
> > index fd13166..33810b6 100644
> > --- a/mm/sparse.c
> > +++ b/mm/sparse.c
> > @@ -256,6 +256,10 @@ void __init memblocks_present(void)
> > 	struct memblock_region *reg;
> > 
> > 	for_each_memblock(memory, reg) {
> > +
> > +		if (memblock_is_nomap(reg))
> > +			continue;
> > +
> > 		memory_present(memblock_get_region_node(reg),
> > 			       memblock_region_memory_base_pfn(reg),
> > 			       memblock_region_memory_end_pfn(reg));
> 
> 
> The logic looks good, while I am not sure this would take effect. Since the
> metadata is SECTION size aligned while memblock is not.
> 
> If I am correct, on arm64, we mark nomap memblock in map_mem()
> 
>     memblock_mark_nomap(kernel_start, kernel_end - kernel_start);

The nomap is also done by EFI code in ${src}/drivers/firmware/efi/arm-init.c

.. and hopefully in the future by this:
https://lkml.org/lkml/2019/7/12/126

So it is not really striclty associated with the map_mem().

So it is extremely dependent on the platform how much memory will end up mapped 
as nomap.

> 
> And kernel text area is less than 40M, if I am right. This means
> memblocks_present would still mark the section present. 
> 
> Would you mind showing how much memory range it is marked nomap?

We actually have some downstream patches that are using this nomap flag for
more than the use-cases I described above which would enflate the nomap regions 
a bit :)

> 
> > 
> > -- 
> > 2.7.4
> 



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