Re: [PATCH v4 1/2] x86/setup: always add the beginning of RAM as memblock.memory

From: Mike Rapoport <rppt@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Andrea Arcangeli" <aarcange@redhat.com>,
	"Baoquan He" <bhe@redhat.com>, "Borislav Petkov" <bp@alien8.de>,
	"Chris Wilson" <chris@chris-wilson.co.uk>,
	"David Hildenbrand" <david@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Łukasz Majczak" <lma@semihalf.com>,
	"Mel Gorman" <mgorman@suse.de>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Mike Rapoport" <rppt@linux.ibm.com>, "Qian Cai" <cai@lca.pw>,
	"Sarvela, Tomi P" <tomi.p.sarvela@intel.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>, stable <stable@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>
Subject: Re: [PATCH v4 1/2] x86/setup: always add the beginning of RAM as memblock.memory
Date: Sun, 31 Jan 2021 10:03:56 +0200	[thread overview]
Message-ID: <20210131080356.GE242749@kernel.org> (raw)
In-Reply-To: <CAHk-=wjJLdjqN2W_hwUmYCM8u=1tWnKsm46CYfdKPP__anGvJw@mail.gmail.com>

On Sat, Jan 30, 2021 at 04:37:54PM -0800, Linus Torvalds wrote:
> On Sat, Jan 30, 2021 at 2:10 PM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > In either case, e820__memblock_setup() won't add the range 0x0000 - 0x1000
> > to memblock.memory and later during memory map initialization this range is
> > left outside any zone.
> 
> Honestly, this just sounds like memblock being stupid in the first place.
> 
> Why aren't these zones padded to sane alignments?

The implicit alignment of zones would be a guess. What alignment would be
sane here? 1M? MAX_ORDER? pageblock_order?

I'm not sure that if an architecture reports its memory at X and we use,
say, round_down(X, 1M) for node[0]->node_start_pfn and
zone[0]->zone_start_pfn it wouldn't cause boot failure on some system out
there in the wild.

> This patch smells like working around the memblock code being fragile
> rather than a real fix.
>
> That's *particularly* true when the very line above it did a
> "memblock_reserve()" of the exact same range that the memblock_add()
> "adds".

The most correct thing to do would have been to 

	memblock_add(0, end_of_first_memory_bank);

Somewhere at e820__memblock_setup().

But that would mean we also must change the way e820__memblock_setup()
reserves memory and that seemed to me like really asking for troubles so
I've limited the registration of memory to the range that's for sure
reserved.

A part of the problem is that x86 adds only usable memory to
memblock.memory omitting holes and reserved areas, while free_area_init()
presumes that memblock.memory covers populated physical memory.

I've tried implicitly adding ranges from memblock.reserved to
memblock.memory if they were not there and it had broken some arm machines:

https://lore.kernel.org/lkml/127999c4-7d56-0c36-7f88-8e1a5c934cae@collabora.com

I do feel that free_area_init() is fragile and no doubt there is a room for
improvement there. But I think the safer way forward is to reduce
inconsistencies between arch and generic code, so that we won't need to
guess what is the memory layout at free_area_init() time.

>               Linus

-- 
Sincerely yours,
Mike.