Re: [PATCH] memblock: config the number of init memblock regions

[Mike Rapoport]

On Tue, May 10, 2022 at 06:55:23PM -0700, Andrew Morton wrote:
> On Wed, 11 May 2022 01:05:30 +0000 Zhou Guanghui <zhouguanghui1@xxxxxxxxxx> wrote:
> 
> > During early boot, the number of memblocks may exceed 128(some memory
> > areas are not reported to the kernel due to test failures. As a result,
> > contiguous memory is divided into multiple parts for reporting). If
> > the size of the init memblock regions is exceeded before the array size
> > can be resized, the excess memory will be lost.

I'd like to see more details about how firmware creates that sparse memory
map in the changelog.

> > 
> > ...
> >
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -89,6 +89,14 @@ config SPARSEMEM_VMEMMAP
> >  	  pfn_to_page and page_to_pfn operations.  This is the most
> >  	  efficient option when sufficient kernel resources are available.
> >  
> > +config MEMBLOCK_INIT_REGIONS
> > +	int "Number of init memblock regions"
> > +	range 128 1024
> > +	default 128
> > +	help
> > +	  The number of init memblock regions which used to track "memory" and
> > +	  "reserved" memblocks during early boot.
> > +
> >  config HAVE_MEMBLOCK_PHYS_MAP
> >  	bool
> >  
> > diff --git a/mm/memblock.c b/mm/memblock.c
> > index e4f03a6e8e56..6893d26b750e 100644
> > --- a/mm/memblock.c
> > +++ b/mm/memblock.c
> > @@ -22,7 +22,7 @@
> >  
> >  #include "internal.h"
> >  
> > -#define INIT_MEMBLOCK_REGIONS			128
> > +#define INIT_MEMBLOCK_REGIONS			CONFIG_MEMBLOCK_INIT_REGIONS
> 
> Consistent naming would be nice - MEMBLOCK_INIT versus INIT_MEMBLOCK.
> 
> Can we simply increase INIT_MEMBLOCK_REGIONS to 1024 and avoid the
> config option?  It appears that the overhead from this would be 60kB or
> so.

60k is not big, but using 1024 entries array for 2-4 memory banks on
systems that don't report that fragmented memory map is really a waste.

We can make this per platform opt-in, like INIT_MEMBLOCK_RESERVED_REGIONS ...

> Or zero if CONFIG_ARCH_KEEP_MEMBLOCK and CONFIG_MEMORY_HOTPLUG
> are cooperating.

... or add code that will discard unused parts of memblock arrays even if
CONFIG_ARCH_KEEP_MEMBLOCK=y.

-- 
Sincerely yours,
Mike.