linux-kernel - Re: [PATCH v2] memblock: add no-map alloc functions

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Message-ID: <ZiFgYWydIwvnpIIY@kernel.org>
Date: Thu, 18 Apr 2024 21:03:13 +0300
From: Mike Rapoport <rppt@...nel.org>
To: DaeRo Lee <skseofh@...il.com>
Cc: robh@...nel.org, saravanak@...gle.com, akpm@...ux-foundation.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Daero Lee <daero_le.lee@...sung.com>
Subject: Re: [PATCH v2] memblock: add no-map alloc functions

On Thu, Apr 18, 2024 at 11:54:15PM +0900, DaeRo Lee wrote:
> 2024년 4월 17일 (수) 오후 3:03, Mike Rapoport <rppt@...nel.org>님이 작성:
> >
> > On Tue, Apr 16, 2024 at 09:06:35PM +0900, skseofh@...il.com wrote:
> > > From: Daero Lee <daero_le.lee@...sung.com>
> > >
> > > Like reserved-memory with the 'no-map' property and only 'size' property
> > > (w/o 'reg' property), there are memory regions need to be allocated in
> > > memblock.memory marked with the MEMBLOCK_NOMAP flag, but should not be
> > > allocated in memblock.reserved.
> >
> > This still does not explain why you need such regions.
> >
> > As Wei Yang explained, memblock does not allocate memory from
> > memblock.reserved. The memblock.reserved array represents memory that is in
> > use by firmware or by early kernel allocations and cannot be freed to page
> > allocator.
> Thank you for your comments. I used the wrong word.
> When I use 'allocate', I mean that the region 'adds' to the memblock.reserved.
> 
> >
> > If you have a region that's _NOMAP in memblock.memory and is absent in
> > memblock.reserved it will not be mapped by the kernel page tables, but it
> > will be considered as free memory by the core mm.
> >
> > Is this really what you want?
> If my understanding is right, before freeing (memory && !reserved)
> area, we marked the memblock.reserved regions and memblock.memory
> regions with no-map flag. And when we free (memory && !reserved) area,
> we skip the memblock.memory regions with no-map(see
> should_skip_region). So, I think that the memory regions with no-map
> flag will not be considered as free memory.

You are right here.

But I still don't understand *why* do you want to change the way
early_init_dt_alloc_reserved_memory_arch() works.
 
> Regards,
> DaeRo Lee

-- 
Sincerely yours,
Mike.