linux-kernel - Re: [PATCH v8] arm64: mm: Populate vmemmap at the page level if not section aligned

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Message-ID: <b68799da-7290-418a-b7db-4cfad7b12e0f@quicinc.com>
Date: Tue, 4 Mar 2025 15:25:55 +0800
From: Zhenhua Huang <quic_zhenhuah@...cinc.com>
To: David Hildenbrand <david@...hat.com>, <anshuman.khandual@....com>,
        <catalin.marinas@....com>
CC: <will@...nel.org>, <ardb@...nel.org>, <ryan.roberts@....com>,
        <mark.rutland@....com>, <joey.gouly@....com>,
        <dave.hansen@...ux.intel.com>, <akpm@...ux-foundation.org>,
        <chenfeiyang@...ngson.cn>, <chenhuacai@...nel.org>,
        <linux-mm@...ck.org>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <quic_tingweiz@...cinc.com>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH v8] arm64: mm: Populate vmemmap at the page level if not
 section aligned



On 2025/3/3 18:01, David Hildenbrand wrote:
> On 19.02.25 09:40, Zhenhua Huang wrote:
>> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
>> to 27, making one section 128M. The related page struct which vmemmap
>> points to is 2M then.
>> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
>> vmemmap to populate at the PMD section level which was suitable
>> initially since hot plug granule is always one section(128M). However,
>> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
>> existing arm64 assumptions.
>>
>> The first problem is that if start or end is not aligned to a section
>> boundary, such as when a subsection is hot added, populating the entire
>> section is wasteful.
>>
>> The next problem is if we hotplug something that spans part of 128 MiB
>> section (subsections, let's call it memblock1), and then hotplug 
>> something
>> that spans another part of a 128 MiB section(subsections, let's call it
>> memblock2), and subsequently unplug memblock1, vmemmap_free() will clear
>> the entire PMD entry which also supports memblock2 even though memblock2
>> is still active.
>>
>> Assuming hotplug/unplug sizes are guaranteed to be symmetric. Do the
>> fix similar to x86-64: populate to pages levels if start/end is not 
>> aligned
>> with section boundary.
>>
>> Cc: <stable@...r.kernel.org> # v5.4+
>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@...cinc.com>
>> ---
>> Hi Catalin and David,
>> Following our latest discussion, I've updated the patch for your review.
>> I also removed Catalin's review tag since I've made significant 
>> modifications.
>>   arch/arm64/mm/mmu.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index b4df5bc5b1b8..de05ccf47f21 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1177,8 +1177,11 @@ int __meminit vmemmap_populate(unsigned long 
>> start, unsigned long end, int node,
>>           struct vmem_altmap *altmap)
>>   {
>>       WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
>> +    /* [start, end] should be within one section */
>> +    WARN_ON(end - start > PAGES_PER_SECTION * sizeof(struct page));
>> -    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
>> +    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
>> +        (end - start < PAGES_PER_SECTION * sizeof(struct page)))
> 
> Indentation should be
> 
>      if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
>          (end - start < PAGES_PER_SECTION * sizeof(struct page)))
> 

Thanks, I will repost with the above fix and WARN_ON_ONCE as you 
preferred in v7.

> 
> Acked-by: David Hildenbrand <david@...hat.com>
> 
> 
> Thanks!
>