Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB | Date | Wed, 19 May 2021 14:03:23 +0200 |
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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 <songmuchun@bytedance.com> >> --- >> 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 <linux/mm.h> >> #include <linux/vmalloc.h> >> #include <linux/set_memory.h> >> +#include <linux/hugetlb.h> >> >> #include <asm/barrier.h> >> #include <asm/cputype.h> >> @@ -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.
vmemmap access (accessing the memmap via a virtual address) itself is not an issue. Manually walking (vmemmap) page tables might behave differently, not sure if ptdump would require any synchronization.
> > Does not this vmemmap operation increase latency for HugeTLB usage ? > Should not this runtime enablement also take into account some other > qualifying information apart from potential memory save from struct > page areas. Just wondering.
That's one of the reasons why it explicitly has to be enabled by an admin.
-- Thanks,
David / dhildenb
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