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[91.12.99.21]) by smtp.gmail.com with ESMTPSA id v15sm8757755wmj.39.2021.05.20.04.59.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 May 2021 04:59:28 -0700 (PDT) Subject: Re: [PATCH] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB To: Anshuman Khandual , Muchun Song , will@kernel.org, akpm@linux-foundation.org, bodeddub@amazon.com, osalvador@suse.de, mike.kravetz@oracle.com, rientjes@google.com Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, duanxiongchun@bytedance.com, fam.zheng@bytedance.com, zhengqi.arch@bytedance.com References: <20210518091826.36937-1-songmuchun@bytedance.com> <1b9d008a-7544-cc85-5c2f-532b984eb5b5@arm.com> <88114091-fbb2-340d-b69b-a572fa340265@redhat.com> From: David Hildenbrand Organization: Red Hat Message-ID: <45c1a368-3d31-e92d-f120-4dca0eb2111d@redhat.com> Date: Thu, 20 May 2021 13:59:28 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 20.05.21 13:54, Anshuman Khandual wrote: > > On 5/19/21 5:33 PM, David Hildenbrand wrote: >> On 19.05.21 13:45, Anshuman Khandual wrote: >>> >>> >>> On 5/18/21 2:48 PM, Muchun Song wrote: >>>> The preparation of supporting freeing vmemmap associated with each >>>> HugeTLB page is ready, so we can support this feature for arm64. >>>> >>>> Signed-off-by: Muchun Song >>>> --- >>>>   arch/arm64/mm/mmu.c | 5 +++++ >>>>   fs/Kconfig          | 2 +- >>>>   2 files changed, 6 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>> index 5d37e461c41f..967b01ce468d 100644 >>>> --- a/arch/arm64/mm/mmu.c >>>> +++ b/arch/arm64/mm/mmu.c >>>> @@ -23,6 +23,7 @@ >>>>   #include >>>>   #include >>>>   #include >>>> +#include >>>>     #include >>>>   #include >>>> @@ -1134,6 +1135,10 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, >>>>       pmd_t *pmdp; >>>>         WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); >>>> + >>>> +    if (is_hugetlb_free_vmemmap_enabled() && !altmap) >>>> +        return vmemmap_populate_basepages(start, end, node, altmap); >>> >>> Not considering the fact that this will force the kernel to have only >>> base page size mapping for vmemmap (unless altmap is also requested) >>> which might reduce the performance, it also enables vmemmap mapping to >>> be teared down or build up at runtime which could potentially collide >>> with other kernel page table walkers like ptdump or memory hotremove >>> operation ! How those possible collisions are protected right now ? >> >> Hi Anshuman, >> >> Memory hotremove is not an issue IIRC. At the time memory is removed, all huge pages either have been migrated away or dissolved; the vmemmap is stable. > > But what happens when a hot remove section's vmemmap area (which is being > teared down) is nearby another vmemmap area which is either created or > being destroyed for HugeTLB alloc/free purpose. As you mentioned HugeTLB > pages inside the hot remove section might be safe. But what about other > HugeTLB areas whose vmemmap area shares page table entries with vmemmap > entries for a section being hot removed ? Massive HugeTLB alloc/use/free > test cycle using memory just adjacent to a memory hotplug area, which is > always added and removed periodically, should be able to expose this problem. > > IIUC unlike vmalloc(), vmemap mapping areas in the kernel page table were > always constant unless there are hotplug add or remove operations which > are protected with a hotplug lock. Now with this change, we could have > simultaneous walking and add or remove of the vmemap areas without any > synchronization. Is not this problematic ? > > On arm64 memory hot remove operation empties free portions of the vmemmap > table after clearing them. Hence all concurrent walkers (hugetlb_vmemmap, > hot remove, ptdump etc) need to be synchronized against hot remove. > > From arch/arm64/mm/mmu.c > > void vmemmap_free(unsigned long start, unsigned long end, > struct vmem_altmap *altmap) > { > #ifdef CONFIG_MEMORY_HOTPLUG > WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); > > unmap_hotplug_range(start, end, true, altmap); > free_empty_tables(start, end, VMEMMAP_START, VMEMMAP_END); > #endif > } You are right, however, AFAIR 1) We always populate base pages, meaning we only modify PTEs and not actually add/remove page tables when creating/destroying a hugetlb page. Page table walkers should be fine and not suddenly run into a use-after-free. 2) For pfn_to_page() users to never fault, we have to do an atomic exchange of PTES, meaning, someone traversing a page table looking for pte_none() entries (like free_empty_tables() in your example) should never get a false positive. Makes sense, or am I missing something? > >> >> vmemmap access (accessing the memmap via a virtual address) itself is not an issue. Manually walking (vmemmap) page tables might behave > > Right. > > differently, not sure if ptdump would require any synchronization. > > Dumping an wrong value is probably okay but crashing because a page table > entry is being freed after ptdump acquired the pointer is bad. On arm64, > ptdump() is protected against hotremove via [get|put]_online_mems(). Okay, and as the feature in question only exchanges PTEs, we should be fine. -- Thanks, David / dhildenb