Re: [PATCH] arm64: mm: hugetlb: add support for free vmemmap pages of HugeTLB

[David Hildenbrand]

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@xxxxxxxxxxxxx>
> > ---
> >   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