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

[Wei Yang]

On Sat, Jul 13, 2019 at 01:53:25PM +0000, Raslan, KarimAllah wrote:
>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@xxxxxxxxxxxxxxxxxxxx>
>> > Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
>> > Cc: Oscar Salvador <osalvador@xxxxxxx>
>> > Cc: Michal Hocko <mhocko@xxxxxxxx>
>> > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
>> > Cc: Baoquan He <bhe@xxxxxxxxxx>
>> > Cc: Qian Cai <cai@xxxxxx>
>> > Cc: Wei Yang <richard.weiyang@xxxxxxxxx>
>> > Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
>> > Cc: linux-mm@xxxxxxxxx
>> > Cc: linux-kernel@xxxxxxxxxxxxxxx
>> > Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
>> > ---
>> > 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 :)
>

Thanks for your explanation.

If my understanding is correct, the range you mark nomap could not be used by
the system, it looks those ranges are useless for the system. Just curious
about how linux could use these memory after marking nomap?

>> 
>> > 
>> > -- 
>> > 2.7.4
>> 
>
>
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-- 
Wei Yang
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