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

[Anshuman Khandual]

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);
> +
>  	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.

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)