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Mon, 20 Jun 2022 01:29:15 -0700 (PDT) Date: Mon, 20 Jun 2022 16:29:11 +0800 From: Muchun Song To: David Hildenbrand Cc: akpm@linux-foundation.org, corbet@lwn.net, mike.kravetz@oracle.com, osalvador@suse.de, paulmck@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, duanxiongchun@bytedance.com, smuchun@gmail.com Subject: Re: [PATCH v4 2/2] mm: memory_hotplug: make hugetlb_optimize_vmemmap compatible with memmap_on_memory Message-ID: References: <20220619133851.68184-1-songmuchun@bytedance.com> <20220619133851.68184-3-songmuchun@bytedance.com> <226243a9-b4f5-182e-1a5b-7b8d5c28f3b3@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <226243a9-b4f5-182e-1a5b-7b8d5c28f3b3@redhat.com> ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1655713758; a=rsa-sha256; cv=none; b=e6u4wYvEGQCvga/d7dIVU170vNUMW3fI2hB3yZ96r6CXo869oX1TogeM3ujpi26tycVlVL xGS44VeNru8sWN/NOOFBV1rdVWv9AqqN5eL11Qm+efwA8Tq/pguvnc8ZXshVkKckpTTHI8 IXZw5Vj5+lXV2klcK10rxdQq7lKRQKU= ARC-Authentication-Results: i=1; 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dmarc=pass (policy=none) header.from=bytedance.com; spf=pass (imf05.hostedemail.com: domain of songmuchun@bytedance.com designates 209.85.214.169 as permitted sender) smtp.mailfrom=songmuchun@bytedance.com X-Stat-Signature: kck4rh7bcipq55pzhops7jkzp31h7buo X-Rspamd-Queue-Id: 6E1BD100007 X-HE-Tag: 1655713757-610433 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 Mon, Jun 20, 2022 at 09:47:22AM +0200, David Hildenbrand wrote: > On 20.06.22 09:22, Muchun Song wrote: > > On Sun, Jun 19, 2022 at 09:38:51PM +0800, Muchun Song wrote: > >> For now, the feature of hugetlb_free_vmemmap is not compatible with the > >> feature of memory_hotplug.memmap_on_memory, and hugetlb_free_vmemmap > >> takes precedence over memory_hotplug.memmap_on_memory. However, someone > >> wants to make memory_hotplug.memmap_on_memory takes precedence over > >> hugetlb_free_vmemmap since memmap_on_memory makes it more likely to > >> succeed memory hotplug in close-to-OOM situations. So the decision > >> of making hugetlb_free_vmemmap take precedence is not wise and elegant. > >> The proper approach is to have hugetlb_vmemmap.c do the check whether > >> the section which the HugeTLB pages belong to can be optimized. If > >> the section's vmemmap pages are allocated from the added memory block > >> itself, hugetlb_free_vmemmap should refuse to optimize the vmemmap, > >> otherwise, do the optimization. Then both kernel parameters are > >> compatible. So this patch introduces VmemmapSelfHosted to mask any > >> non-optimizable vmemmap pages. The hugetlb_vmemmap can use this flag > >> to detect if a vmemmap page can be optimized. > >> > >> Signed-off-by: Muchun Song > >> Co-developed-by: Oscar Salvador > >> Signed-off-by: Oscar Salvador > >> --- > >> Documentation/admin-guide/kernel-parameters.txt | 22 +++++------ > >> Documentation/admin-guide/sysctl/vm.rst | 5 +-- > >> include/linux/memory_hotplug.h | 9 ----- > >> include/linux/page-flags.h | 11 ++++++ > >> mm/hugetlb_vmemmap.c | 52 +++++++++++++++++++++---- > >> mm/memory_hotplug.c | 27 ++++++------- > >> 6 files changed, 79 insertions(+), 47 deletions(-) > >> > >> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > >> index 8090130b544b..d740e2ed0e61 100644 > >> --- a/Documentation/admin-guide/kernel-parameters.txt > >> +++ b/Documentation/admin-guide/kernel-parameters.txt > >> @@ -1722,9 +1722,11 @@ > >> Built with CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON=y, > >> the default is on. > >> > >> - This is not compatible with memory_hotplug.memmap_on_memory. > >> - If both parameters are enabled, hugetlb_free_vmemmap takes > >> - precedence over memory_hotplug.memmap_on_memory. > >> + Note that the vmemmap pages may be allocated from the added > >> + memory block itself when memory_hotplug.memmap_on_memory is > >> + enabled, those vmemmap pages cannot be optimized even if this > >> + feature is enabled. Other vmemmap pages not allocated from > >> + the added memory block itself do not be affected. > >> > >> hung_task_panic= > >> [KNL] Should the hung task detector generate panics. > >> @@ -3069,10 +3071,12 @@ > >> [KNL,X86,ARM] Boolean flag to enable this feature. > >> Format: {on | off (default)} > >> When enabled, runtime hotplugged memory will > >> - allocate its internal metadata (struct pages) > >> - from the hotadded memory which will allow to > >> - hotadd a lot of memory without requiring > >> - additional memory to do so. > >> + allocate its internal metadata (struct pages, > >> + those vmemmap pages cannot be optimized even > >> + if hugetlb_free_vmemmap is enabled) from the > >> + hotadded memory which will allow to hotadd a > >> + lot of memory without requiring additional > >> + memory to do so. > >> This feature is disabled by default because it > >> has some implication on large (e.g. GB) > >> allocations in some configurations (e.g. small > >> @@ -3082,10 +3086,6 @@ > >> Note that even when enabled, there are a few cases where > >> the feature is not effective. > >> > >> - This is not compatible with hugetlb_free_vmemmap. If > >> - both parameters are enabled, hugetlb_free_vmemmap takes > >> - precedence over memory_hotplug.memmap_on_memory. > >> - > >> memtest= [KNL,X86,ARM,M68K,PPC,RISCV] Enable memtest > >> Format: > >> default : 0 > >> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst > >> index 5c9aa171a0d3..d7374a1e8ac9 100644 > >> --- a/Documentation/admin-guide/sysctl/vm.rst > >> +++ b/Documentation/admin-guide/sysctl/vm.rst > >> @@ -565,9 +565,8 @@ See Documentation/admin-guide/mm/hugetlbpage.rst > >> hugetlb_optimize_vmemmap > >> ======================== > >> > >> -This knob is not available when memory_hotplug.memmap_on_memory (kernel parameter) > >> -is configured or the size of 'struct page' (a structure defined in > >> -include/linux/mm_types.h) is not power of two (an unusual system config could > >> +This knob is not available when the size of 'struct page' (a structure defined > >> +in include/linux/mm_types.h) is not power of two (an unusual system config could > >> result in this). > >> > >> Enable (set to 1) or disable (set to 0) the feature of optimizing vmemmap pages > >> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h > >> index 20d7edf62a6a..e0b2209ab71c 100644 > >> --- a/include/linux/memory_hotplug.h > >> +++ b/include/linux/memory_hotplug.h > >> @@ -351,13 +351,4 @@ void arch_remove_linear_mapping(u64 start, u64 size); > >> extern bool mhp_supports_memmap_on_memory(unsigned long size); > >> #endif /* CONFIG_MEMORY_HOTPLUG */ > >> > >> -#ifdef CONFIG_MHP_MEMMAP_ON_MEMORY > >> -bool mhp_memmap_on_memory(void); > >> -#else > >> -static inline bool mhp_memmap_on_memory(void) > >> -{ > >> - return false; > >> -} > >> -#endif > >> - > >> #endif /* __LINUX_MEMORY_HOTPLUG_H */ > >> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > >> index e66f7aa3191d..2aa5dcbfe468 100644 > >> --- a/include/linux/page-flags.h > >> +++ b/include/linux/page-flags.h > >> @@ -193,6 +193,11 @@ enum pageflags { > >> > >> /* Only valid for buddy pages. Used to track pages that are reported */ > >> PG_reported = PG_uptodate, > >> + > >> +#ifdef CONFIG_MEMORY_HOTPLUG > >> + /* For self-hosted memmap pages */ > >> + PG_vmemmap_self_hosted = PG_owner_priv_1, > >> +#endif > >> }; > >> > >> #define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1) > >> @@ -628,6 +633,12 @@ PAGEFLAG_FALSE(SkipKASanPoison, skip_kasan_poison) > >> */ > >> __PAGEFLAG(Reported, reported, PF_NO_COMPOUND) > >> > >> +#ifdef CONFIG_MEMORY_HOTPLUG > >> +PAGEFLAG(VmemmapSelfHosted, vmemmap_self_hosted, PF_ANY) > >> +#else > >> +PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted) > >> +#endif > >> + > >> /* > >> * On an anonymous page mapped into a user virtual memory area, > >> * page->mapping points to its anon_vma, not to a struct address_space; > >> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c > >> index 1089ea8a9c98..73bfbb47f6a4 100644 > >> --- a/mm/hugetlb_vmemmap.c > >> +++ b/mm/hugetlb_vmemmap.c > >> @@ -10,7 +10,7 @@ > >> */ > >> #define pr_fmt(fmt) "HugeTLB: " fmt > >> > >> -#include > >> +#include > >> #include "hugetlb_vmemmap.h" > >> > >> /* > >> @@ -97,18 +97,54 @@ int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head) > >> return ret; > >> } > >> > >> +static unsigned int vmemmap_optimizable_pages(struct hstate *h, > >> + struct page *head) > >> +{ > >> + if (READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF) > >> + return 0; > >> + > >> + if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) { > >> + unsigned long pfn = page_to_pfn(head); > >> + > >> + /* > >> + * Due to HugeTLB alignment requirements and the vmemmap pages > >> + * being at the start of the hotplugged memory region in > >> + * memory_hotplug.memmap_on_memory case. Checking the first > >> + * vmemmap page's vmemmap if it is marked as VmemmapSelfHosted > >> + * is sufficient. > >> + * > >> + * [ hotplugged memory ] > >> + * [ section ][...][ section ] > >> + * [ vmemmap ][ usable memory ] > >> + * ^ | | | > >> + * +---+ | | > >> + * ^ | | > >> + * +-------+ | > >> + * ^ | > >> + * +-------------------------------------------+ > >> + * > >> + * Hotplugged memory block never has non-present sections, while > >> + * boot memory block can have one or more. So pfn_valid() is > >> + * used to filter out the non-present section which also cannot > >> + * be memmap_on_memory. > >> + */ > >> + pfn = ALIGN_DOWN(pfn, PHYS_PFN(memory_block_size_bytes())); > >> + if (pfn_valid(pfn) && PageVmemmapSelfHosted(pfn_to_page(pfn))) > > > > Although it works, I think PageVmemmapSelfHosted() check for the 1st pfn's > > vmemmap page is not always reliable. Since we reused PG_owner_priv_1 > > as PG_vmemmap_self_hosted, the test is noly reliable for vmemmap page's > > vmemmap page. Other non-vmemmap page can be flagged with PG_owner_priv_1. > > So this check can be false-positive. Maybe the following code snippet is > > the solution. > > How could that happen for pages used for backing a vmemmap? > It cannot happen for memmap_on_memory case. Howwver, it can happen for other cases. E.g. the 1st pfn (of boot memory block) whose vmemmap page may be flagged as PG_owner_priv_1 (if PG_swapcache is set). Then, the check is false-positive. > > > > Any thoughts? Oscar or David. > > First of all, I think you should really avoid using > memory_block_size_bytes(); when using memory_block_size_bytes(), you > wouldn't need PageVmemmapSelfHosted(), you can just check if the vmemmap > of the page is itself. But I think we should try making this independent > of the memory block size. > Agree. > If virt_to_page(page) doesn't work, maybe just traverse the direct map > to find the page backing page directly? > Yeah, now I have tried to walk page tables to get the backing page. I'll update a new version. Thanks. > -- > Thanks, > > David / dhildenb > >