linux-kernel - Re: [PATCH] mm: memblock: avoid to create memmap for memblock nomap regions

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Date:   Thu, 5 May 2022 09:54:00 -0700
From:   Mike Rapoport <rppt@...nel.org>
To:     Faiyaz Mohammed <quic_faiyazm@...cinc.com>
Cc:     quic_vjitta@...cinc.com, karahmed@...zon.de, qperret@...gle.com,
        robh@...nel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        frowand.list@...il.com, devicetree@...r.kernel.org
Subject: Re: [PATCH] mm: memblock: avoid to create memmap for memblock nomap
 regions

On Thu, May 05, 2022 at 08:46:15PM +0530, Faiyaz Mohammed wrote:
> 
> On 4/12/2022 10:56 PM, Mike Rapoport wrote:
> > On Tue, Apr 12, 2022 at 12:39:32AM +0530, Faiyaz Mohammed wrote:
> >> This 'commit 86588296acbf ("fdt: Properly handle "no-map" field in the
> >> memory region")' is keeping the no-map regions in memblock.memory with
> >> MEMBLOCK_NOMAP flag set to use no-map memory for EFI using memblock api's,
> >> but during the initialization sparse_init mark all memblock.memory as
> >> present using for_each_mem_pfn_range, which is creating the memmap for
> >> no-map memblock regions. To avoid it skiping the memblock.memory regions
> >> set with MEMBLOCK_NOMAP set and with this change we will be able to save
> >> ~11MB memory for ~612MB carve out.
> > The MEMBLOCK_NOMAP is very fragile and caused a lot of issues already. I
> > really don't like the idea if adding more implicit assumptions about how
> > NOMAP memory may or may not be used in a generic iterator function.
> 
> Sorry for delayed response.
> Yes, it is possible that implicit assumption can create
> misunderstanding. How about adding command line option and control the
> no-map region in fdt.c driver, to decide whether to keep "no-map" region
> with NOMAP flag or remove?. Something like below

I really don't like memblock_remove() for such cases.
Pretending there is a hole when there is an actual DRAM makes things really
hairy when it comes to memory map and page allocator initialization.
You wouldn't want to trade system stability and random memory corruptions
for 11M of "saved" memory.
 
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -1180,8 +1180,10 @@ int __init __weak
> early_init_dt_reserve_memory_arch(phys_addr_t base,
>                  */
>                 if (memblock_is_region_reserved(base, size))
>                         return -EBUSY;
> -
> -               return memblock_mark_nomap(base, size);
> +               if (remove_nomap_region)
> +                       return memblock_remove(base, size);
> +               else
> +                       return memblock_mark_nomap(base, size);
> Thanks and regards,
> Mohammed Faiyaz
> 

-- 
Sincerely yours,
Mike.