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

[Wei Yang <richard.weiyang@gmail.com>]

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);

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?

>-- 
>2.7.4

-- 
Wei Yang
Help you, Help me