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[2003:cb:c747:ff00:9d85:4afb:a7df:6c45]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-390cd882602sm6196411f8f.41.2025.02.26.09.13.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Feb 2025 09:13:50 -0800 (PST) Message-ID: <00c82c92-35a0-441c-b5b5-e4a6c8a4a9b7@redhat.com> Date: Wed, 26 Feb 2025 18:13:49 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> <871c0dae-c419-4ac2-9472-6901aab90dcf@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sorry, I somehow missed this mail. >>> 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 ... > > Shall I document and WARN_ON if the size exceeds? like: > WARN_ON(end - start > PAGES_PER_SECTION * sizeof(struct page)) Probably WARN_ON_ONCE() along with a comment that we should never exceed a single memory section. > > Since vmemmap_populate() is implemented per architecture, the change > should apply for other architectures as well. However I have no setup to > test it on... > Therefore, May I implement it only for arm64 now ? Would work for me; better than no warning. > Additionally, from previous discussion, the change is worth > backporting(apologies for missing to CC stable kernel in this version). > Keeping it for arm64 should simplify for backporting. WDYT? Jup. Of course, we could add a generic warning in a separate patch. > >> >> >>>> +/* >>>> + * 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? > > It's invoked within vmemmap_populate_hugepages(), which is called by > vmemmap_populate(). This implies that we are always performing a whole > section hotplug? Ah, you meant only in the context of this change, yes. I was confused, because there are other reasons why we run into that fallback (failing to allocate a PMD). -- Cheers, David / dhildenb