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

[Muchun Song]

On Wed, May 19, 2021 at 8:35 PM Anshuman Khandual
<anshuman.khandual at arm.com> 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 at 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);
> > +
> >       do {
> >               next = pmd_addr_end(addr, end);
> >
> > diff --git a/fs/Kconfig b/fs/Kconfig
> > index 6ce6fdac00a3..02c2d3bf1cb8 100644
> > --- a/fs/Kconfig
> > +++ b/fs/Kconfig
> > @@ -242,7 +242,7 @@ config HUGETLB_PAGE
> >
> >  config HUGETLB_PAGE_FREE_VMEMMAP
> >       def_bool HUGETLB_PAGE
> > -     depends on X86_64
> > +     depends on X86_64 || ARM64
> >       depends on SPARSEMEM_VMEMMAP
> >
> >  config MEMFD_CREATE
> >
>
> How does this interact with HugeTLB migration as such which might iterate
> over individual constituent struct pages (overriding the same struct page
> for all tail pages when this feature is enabled). A simple test involving
> madvise(ptr, size, MADV_SOFT_OFFLINE) fails on various HugeTLB page sizes,
> with this patch applied. Although I have not debugged this any further.

It is weird. Actually, I didn't change the behaviour of the page migration.
This feature is default off. If you want to enable this feature, you can pass
"hugetlb_free_vmemmap=on" to the boot cmdline. Do you mean that the
success rate of page migration will decrease when you enable this feature?
The rate will increase if disbale. Right?

Thanks.


>
> Soft offlining pfn 0x101c00 at process virtual address 0xffff7fa00000
> soft offline: 0x101c00: hugepage migration failed 1, type bfffc0000010006
>               (referenced|uptodate|head|node=0|zone=2|lastcpupid=0xffff)
>