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[2003:cb:c739:900:900f:3c9e:2f7b:5d0a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4395a04f208sm153583685e9.6.2025.02.17.06.30.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 17 Feb 2025 06:30:25 -0800 (PST) Message-ID: <871c0dae-c419-4ac2-9472-6901aab90dcf@redhat.com> Date: Mon, 17 Feb 2025 15:30:23 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7] arm64: mm: Populate vmemmap at the page level if not section aligned To: Zhenhua Huang , anshuman.khandual@arm.com, catalin.marinas@arm.com Cc: will@kernel.org, ardb@kernel.org, ryan.roberts@arm.com, mark.rutland@arm.com, joey.gouly@arm.com, dave.hansen@linux.intel.com, akpm@linux-foundation.org, chenfeiyang@loongson.cn, chenhuacai@kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, quic_tingweiz@quicinc.com References: <20250217092907.3474806-1-quic_zhenhuah@quicinc.com> <8c1578ed-cfef-4fba-a334-ebf5eac26d60@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: C0244140002 X-Stat-Signature: gekgcqejyfwx6k3x8o4dmbsik14htp7b X-HE-Tag: 1739802629-146911 X-HE-Meta: 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 T48EEuny 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 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: List-Subscribe: List-Unsubscribe: On 17.02.25 11:34, Zhenhua Huang wrote: > > > On 2025/2/17 17:44, David Hildenbrand wrote: >> On 17.02.25 10:29, 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. >>> >>> Signed-off-by: Zhenhua Huang >>> --- >>>   arch/arm64/mm/mmu.c | 3 ++- >>>   1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>> index b4df5bc5b1b8..eec1666da368 100644 >>> --- a/arch/arm64/mm/mmu.c >>> +++ b/arch/arm64/mm/mmu.c >>> @@ -1178,7 +1178,8 @@ int __meminit vmemmap_populate(unsigned long >>> start, unsigned long end, int node, >>>   { >>>       WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); >>> -    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES)) >>> +    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || >>> +        (end - start < PAGES_PER_SECTION * sizeof(struct page))) >>>           return vmemmap_populate_basepages(start, end, node, altmap); >>>       else >>>           return vmemmap_populate_hugepages(start, end, node, altmap); >> >> Yes, this does mimic what x86 does. That handling does look weird, >> because it >> doesn't care about any address alignments, only about the size, which is >> odd. >> >> I wonder if we could do better and move this handling >> into vmemmap_populate_hugepages(), where we already have a fallback >> to vmemmap_populate_basepages(). > > Hi David, > > I had the same doubt initially. > After going through the codes, I noticed for vmemmap_populate(), the > arguments "start" and "end" passed down should already be within one > section. > early path: > for_each_present_section_nr > __populate_section_memmap > .. > vmemmap_populate() > > hotplug path: > __add_pages > section_activate > vmemmap_populate() > > Therefore.. focusing only on the size seems OK to me, and fall back > solution below appears unnecessary? Ah, in that case it is fine. Might make sense to document/enforce that somehow for the time being ... >> +/* >> + * Try to populate PMDs, but fallback to populating base pages when ranges >> + * would only partially cover a PMD. >> + */ >>  int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned >> long end, >>                                          int node, struct vmem_altmap >> *altmap) >>  { >> @@ -313,6 +317,9 @@ int __meminit vmemmap_populate_hugepages(unsigned >> long start, unsigned long end, >>         for (addr = start; addr < end; addr = next) { > > This for loop appears to be redundant for arm64 as well, as above > mentioned, a single call to pmd_addr_end() should suffice. Right, that was what was confusing me in the first place. > >>                 next = pmd_addr_end(addr, end); >> >> +               if (!IS_ALIGNED(addr, PMD_SIZE) || !IS_ALIGNED(next, >> PMD_SIZE)) >> +                       goto fallback; >> + >>                 pgd = vmemmap_pgd_populate(addr, node); >>                 if (!pgd) >>                         return -ENOMEM; >> @@ -346,6 +353,7 @@ int __meminit vmemmap_populate_hugepages(unsigned >> long start, unsigned long end, >>                         } >>                 } else if (vmemmap_check_pmd(pmd, node, addr, next)) >>                         continue; >> +fallback: >>                 if (vmemmap_populate_basepages(addr, next, node, altmap)) >>                         return -ENOMEM; > > It seems we have no chance to call populate_basepages here? Can you elaborate? -- Cheers, David / dhildenb