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[80.187.83.189]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4255d8ab8fdsm33619939f8f.15.2025.10.09.03.27.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Oct 2025 03:27:22 -0700 (PDT) Message-ID: <543e9440-8ee0-4d9e-9b05-0107032d665b@redhat.com> Date: Thu, 9 Oct 2025 12:27:17 +0200 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: (bisected) [PATCH v2 08/37] mm/hugetlb: check for unreasonable folio sizes when registering hstate To: Christophe Leroy , linux-kernel@vger.kernel.org Cc: Zi Yan , Lorenzo Stoakes , "Liam R. Howlett" , Alexander Potapenko , Andrew Morton , Brendan Jackman , Christoph Lameter , Dennis Zhou , Dmitry Vyukov , dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, iommu@lists.linux.dev, io-uring@vger.kernel.org, Jason Gunthorpe , Jens Axboe , Johannes Weiner , John Hubbard , kasan-dev@googlegroups.com, kvm@vger.kernel.org, Linus Torvalds , linux-arm-kernel@axis.com, linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, linux-ide@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mips@vger.kernel.org, linux-mmc@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, Marco Elver , Marek Szyprowski , Michal Hocko , Mike Rapoport , Muchun Song , netdev@vger.kernel.org, Oscar Salvador , Peter Xu , Robin Murphy , Suren Baghdasaryan , Tejun Heo , virtualization@lists.linux.dev, Vlastimil Babka , wireguard@lists.zx2c4.com, x86@kernel.org, "linuxppc-dev@lists.ozlabs.org" References: <20250901150359.867252-1-david@redhat.com> <20250901150359.867252-9-david@redhat.com> <3e043453-3f27-48ad-b987-cc39f523060a@csgroup.eu> <9361c75a-ab37-4d7f-8680-9833430d93d4@redhat.com> <03671aa8-4276-4707-9c75-83c96968cbb2@csgroup.eu> <1db15a30-72d6-4045-8aa1-68bd8411b0ba@redhat.com> <0c730c52-97ee-43ea-9697-ac11d2880ab7@csgroup.eu> 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 On 09.10.25 12:01, Christophe Leroy wrote: > > > Le 09/10/2025 à 11:20, David Hildenbrand a écrit : >> On 09.10.25 11:16, Christophe Leroy wrote: >>> >>> >>> Le 09/10/2025 à 10:14, David Hildenbrand a écrit : >>>> On 09.10.25 10:04, Christophe Leroy wrote: >>>>> >>>>> >>>>> Le 09/10/2025 à 09:22, David Hildenbrand a écrit : >>>>>> On 09.10.25 09:14, Christophe Leroy wrote: >>>>>>> Hi David, >>>>>>> >>>>>>> Le 01/09/2025 à 17:03, David Hildenbrand a écrit : >>>>>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>>>>>> index 1e777cc51ad04..d3542e92a712e 100644 >>>>>>>> --- a/mm/hugetlb.c >>>>>>>> +++ b/mm/hugetlb.c >>>>>>>> @@ -4657,6 +4657,7 @@ static int __init hugetlb_init(void) >>>>>>>>          BUILD_BUG_ON(sizeof_field(struct page, private) * >>>>>>>> BITS_PER_BYTE < >>>>>>>>                  __NR_HPAGEFLAGS); >>>>>>>> +    BUILD_BUG_ON_INVALID(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER); >>>>>>>>          if (!hugepages_supported()) { >>>>>>>>              if (hugetlb_max_hstate || >>>>>>>> default_hstate_max_huge_pages) >>>>>>>> @@ -4740,6 +4741,7 @@ void __init hugetlb_add_hstate(unsigned int >>>>>>>> order) >>>>>>>>          } >>>>>>>>          BUG_ON(hugetlb_max_hstate >= HUGE_MAX_HSTATE); >>>>>>>>          BUG_ON(order < order_base_2(__NR_USED_SUBPAGE)); >>>>>>>> +    WARN_ON(order > MAX_FOLIO_ORDER); >>>>>>>>          h = &hstates[hugetlb_max_hstate++]; >>>>>>>>          __mutex_init(&h->resize_lock, "resize mutex", &h- >>>>>>>>> resize_key); >>>>>>>>          h->order = order; >>>>>> >>>>>> We end up registering hugetlb folios that are bigger than >>>>>> MAX_FOLIO_ORDER. So we have to figure out how a config can trigger >>>>>> that >>>>>> (and if we have to support that). >>>>>> >>>>> >>>>> MAX_FOLIO_ORDER is defined as: >>>>> >>>>> #ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE >>>>> #define MAX_FOLIO_ORDER        PUD_ORDER >>>>> #else >>>>> #define MAX_FOLIO_ORDER        MAX_PAGE_ORDER >>>>> #endif >>>>> >>>>> MAX_PAGE_ORDER is the limit for dynamic creation of hugepages via >>>>> /sys/kernel/mm/hugepages/ but bigger pages can be created at boottime >>>>> with kernel boot parameters without CONFIG_ARCH_HAS_GIGANTIC_PAGE: >>>>> >>>>>      hugepagesz=64m hugepages=1 hugepagesz=256m hugepages=1 >>>>> >>>>> Gives: >>>>> >>>>> HugeTLB: registered 1.00 GiB page size, pre-allocated 0 pages >>>>> HugeTLB: 0 KiB vmemmap can be freed for a 1.00 GiB page >>>>> HugeTLB: registered 64.0 MiB page size, pre-allocated 1 pages >>>>> HugeTLB: 0 KiB vmemmap can be freed for a 64.0 MiB page >>>>> HugeTLB: registered 256 MiB page size, pre-allocated 1 pages >>>>> HugeTLB: 0 KiB vmemmap can be freed for a 256 MiB page >>>>> HugeTLB: registered 4.00 MiB page size, pre-allocated 0 pages >>>>> HugeTLB: 0 KiB vmemmap can be freed for a 4.00 MiB page >>>>> HugeTLB: registered 16.0 MiB page size, pre-allocated 0 pages >>>>> HugeTLB: 0 KiB vmemmap can be freed for a 16.0 MiB page >>>> >>>> I think it's a violation of CONFIG_ARCH_HAS_GIGANTIC_PAGE. The existing >>>> folio_dump() code would not handle it correctly as well. >>> >>> I'm trying to dig into history and when looking at commit 4eb0716e868e >>> ("hugetlb: allow to free gigantic pages regardless of the >>> configuration") I understand that CONFIG_ARCH_HAS_GIGANTIC_PAGE is >>> needed to be able to allocate gigantic pages at runtime. It is not >>> needed to reserve gigantic pages at boottime. >>> >>> What am I missing ? >> >> That CONFIG_ARCH_HAS_GIGANTIC_PAGE has nothing runtime-specific in its >> name. > > In its name for sure, but the commit I mention says: > > On systems without CONTIG_ALLOC activated but that support gigantic > pages, > boottime reserved gigantic pages can not be freed at all. This patch > simply enables the possibility to hand back those pages to memory > allocator. Right, I think it was a historical artifact. > > And one of the hunks is: > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index 7f7fbd8bd9d5b..7a1aa53d188d3 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -19,7 +19,7 @@ config ARM64 > select ARCH_HAS_FAST_MULTIPLIER > select ARCH_HAS_FORTIFY_SOURCE > select ARCH_HAS_GCOV_PROFILE_ALL > - select ARCH_HAS_GIGANTIC_PAGE if CONTIG_ALLOC > + select ARCH_HAS_GIGANTIC_PAGE > select ARCH_HAS_KCOV > select ARCH_HAS_KEEPINITRD > select ARCH_HAS_MEMBARRIER_SYNC_CORE > > So I understand from the commit message that it was possible at that > time to have gigantic pages without ARCH_HAS_GIGANTIC_PAGE as long as > you didn't have to be able to free them during runtime. Yes, I agree. > >> >> Can't we just select CONFIG_ARCH_HAS_GIGANTIC_PAGE for the relevant >> hugetlb config that allows for *gigantic pages*. >> > > We probably can, but I'd really like to understand history and how we > ended up in the situation we are now. > Because blind fixes often lead to more problems. Yes, let's figure out how to to it cleanly. > > If I follow things correctly I see a helper gigantic_page_supported() > added by commit 944d9fec8d7a ("hugetlb: add support for gigantic page > allocation at runtime"). > > And then commit 461a7184320a ("mm/hugetlb: introduce > ARCH_HAS_GIGANTIC_PAGE") is added to wrap gigantic_page_supported() > > Then commit 4eb0716e868e ("hugetlb: allow to free gigantic pages > regardless of the configuration") changed gigantic_page_supported() to > gigantic_page_runtime_supported() > > So where are we now ? In commit fae7d834c43ccdb9fcecaf4d0f33145d884b3e5c Author: Matthew Wilcox (Oracle) Date: Tue Feb 27 19:23:31 2024 +0000 mm: add __dump_folio() We started assuming that a folio in the system (boottime, dynamic, whatever) has a maximum of MAX_FOLIO_NR_PAGES. Any other interpretation doesn't make any sense for MAX_FOLIO_NR_PAGES. So we have two questions: 1) How to teach MAX_FOLIO_NR_PAGES that hugetlb supports gigantic pages 2) How do we handle CONFIG_ARCH_HAS_GIGANTIC_PAGE We have the following options (A) Rename existing CONFIG_ARCH_HAS_GIGANTIC_PAGE to something else that is clearer and add a new CONFIG_ARCH_HAS_GIGANTIC_PAGE. (B) Rename existing CONFIG_ARCH_HAS_GIGANTIC_PAGE -> to something else that is clearer and derive somehow else that hugetlb in that config supports gigantic pages. (c) Just use CONFIG_ARCH_HAS_GIGANTIC_PAGE if hugetlb on an architecture supports gigantic pages. I don't quite see why an architecture should be able to opt in into dynamically allocating+freeing gigantic pages. That's just CONTIG_ALLOC magic and not some arch-specific thing IIRC. Note that in mm/hugetlb.c it is #ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE #ifdef CONFIG_CONTIG_ALLOC Meaning that at least the allocation side is guarded by CONTIG_ALLOC. So I think (C) is just the right thing to do. diff --git a/fs/Kconfig b/fs/Kconfig index 0bfdaecaa8775..12c11eb9279d3 100644 --- a/fs/Kconfig +++ b/fs/Kconfig @@ -283,6 +283,8 @@ config HUGETLB_PMD_PAGE_TABLE_SHARING def_bool HUGETLB_PAGE depends on ARCH_WANT_HUGE_PMD_SHARE && SPLIT_PMD_PTLOCKS +# An architecture must select this option if there is any mechanism (esp. hugetlb) +# could obtain gigantic folios. config ARCH_HAS_GIGANTIC_PAGE bool -- Cheers David / dhildenb