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Peter Anvin" , Ingo Molnar , Linus Torvalds , =?UTF-8?Q?=c5=81ukasz_Majczak?= , Mel Gorman , Michal Hocko , Mike Rapoport , Qian Cai , "Sarvela, Tomi P" , Thomas Gleixner , Vlastimil Babka , linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org, x86@kernel.org References: <20210130221035.4169-1-rppt@kernel.org> <20210130221035.4169-2-rppt@kernel.org> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v4 1/2] x86/setup: always add the beginning of RAM as memblock.memory Message-ID: <56e2c568-b121-8860-a6b0-274ace46d835@redhat.com> Date: Mon, 1 Feb 2021 10:32:44 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <20210130221035.4169-2-rppt@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 30.01.21 23:10, Mike Rapoport wrote: > From: Mike Rapoport >=20 > The physical memory on an x86 system starts at address 0, but this is n= ot > always reflected in e820 map. For example, the BIOS can have e820 entri= es > like >=20 > [ 0.000000] BIOS-provided physical RAM map: > [ 0.000000] BIOS-e820: [mem 0x0000000000001000-0x000000000009ffff] u= sable >=20 > or >=20 > [ 0.000000] BIOS-provided physical RAM map: > [ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x0000000000000fff] r= eserved > [ 0.000000] BIOS-e820: [mem 0x0000000000001000-0x0000000000057fff] u= sable >=20 > In either case, e820__memblock_setup() won't add the range 0x0000 - 0x1= 000 > to memblock.memory and later during memory map initialization this rang= e is > left outside any zone. >=20 > With SPARSEMEM=3Dy there is always a struct page for pfn 0 and this str= uct > page will have it's zone link wrong no matter what value will be set th= ere. >=20 > To avoid this inconsistency, add the beginning of RAM to memblock.memor= y. > Limit the added chunk size to match the reserved memory to avoid > registering memory that may be used by the firmware but never reserved = at > e820__memblock_setup() time. >=20 > Fixes: bde9cfa3afe4 ("x86/setup: don't remove E820_TYPE_RAM for pfn 0") > Signed-off-by: Mike Rapoport > Cc: stable@vger.kernel.org > --- > arch/x86/kernel/setup.c | 8 ++++++++ > 1 file changed, 8 insertions(+) >=20 > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c > index 3412c4595efd..67c77ed6eef8 100644 > --- a/arch/x86/kernel/setup.c > +++ b/arch/x86/kernel/setup.c > @@ -727,6 +727,14 @@ static void __init trim_low_memory_range(void) > * Kconfig help text for X86_RESERVE_LOW. > */ > memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE)); > + > + /* > + * Even if the firmware does not report the memory at address 0 as > + * usable, inform the generic memory management about its existence > + * to ensure it is a part of ZONE_DMA and the memory map for it is > + * properly initialized. > + */ > + memblock_add(0, ALIGN(reserve_low, PAGE_SIZE)); > } > =09 > /* >=20 I think, to make that code more robust, and to not rely on archs to do=20 the right thing, we should do something like 1) Make sure in free_area_init() that each PFN with a memmap (i.e.,=20 falls into a partial present section) is spanned by a zone; that would=20 include PFN 0 in this case. 2) In init_zone_unavailable_mem(), similar to round_up(max_pfn,=20 PAGES_PER_SECTION) handling, consider range [round_down(min_pfn, PAGES_PER_SECTION), min_pfn - 1] which would handle in the x86-64 case [0..0] and, therefore, initialize=20 PFN 0. Also, I think the special-case of PFN 0 is analogous to the=20 round_up(max_pfn, PAGES_PER_SECTION) handling in=20 init_zone_unavailable_mem(): who guarantees that these PFN above the=20 highest present PFN are actually spanned by a zone? I'd suggest going through all zone ranges in free_area_init() first,=20 dealing with zones that have "not section aligned start/end", clamping=20 them up/down if required such that no holes within a section are left=20 uncovered by a zone. --=20 Thanks, David / dhildenb