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

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Date:   Tue, 12 Apr 2022 20:26:36 +0300
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
Subject: Re: [PATCH] mm: memblock: avoid to create memmap for memblock nomap
 regions

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.

-- 
Sincerely yours,
Mike.