Re: [PATCH] x86/mm: synchronize pgd in vmemmap_free()

[John Hubbard <jhubbard@nvidia.com>]

Hi Jerome,

On 05/19/2017 11:01 AM, Jérôme Glisse wrote:
> When we free kernel virtual map we should synchronize p4d/pud for
> all the pgds to avoid any stall entry in non canonical pgd.

"any stale entry in the non-canonical pgd", is what I think you meant to type there.

Also, it would be nice to clarify that commit description a bit: I'm not sure what is meant here by 
a "non-canonical pgd".

Also, it seems like the reshuffling of the internals of sync_global_pgds() deserves at least some 
mention here. More below.

> 
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mel Gorman <mgorman@suse.de>
> ---
>   arch/x86/mm/init_64.c | 17 ++++++++++-------
>   1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index ff95fe8..df753f8 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -108,8 +108,6 @@ void sync_global_pgds(unsigned long start, unsigned long end)
>   		BUILD_BUG_ON(pgd_none(*pgd_ref));
>   		p4d_ref = p4d_offset(pgd_ref, address);
>   
> -		if (p4d_none(*p4d_ref))
> -			continue;
>   
>   		spin_lock(&pgd_lock);
>   		list_for_each_entry(page, &pgd_list, lru) {
> @@ -123,12 +121,16 @@ void sync_global_pgds(unsigned long start, unsigned long end)
>   			pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
>   			spin_lock(pgt_lock);
>   
> -			if (!p4d_none(*p4d_ref) && !p4d_none(*p4d))
> -				BUG_ON(p4d_page_vaddr(*p4d)
> -				       != p4d_page_vaddr(*p4d_ref));
> -
> -			if (p4d_none(*p4d))
> +			if (p4d_none(*p4d_ref)) {
>   				set_p4d(p4d, *p4d_ref);

Is the intention really to set p4d to a zeroed *p4d_ref, or is that a mistake?

> +			} else {
> +				if (!p4d_none(*p4d_ref) && !p4d_none(*p4d))

I think the code needs to be somewhat restructured, but as it stands, the above !p4d_none(*p4d_ref) 
will always be true, because first part of the if/else checked for the opposite case: 
p4d_none(*p4d_ref).  This is a side effect of moving that block of code.

> +					BUG_ON(p4d_page_vaddr(*p4d)
> +					       != p4d_page_vaddr(*p4d_ref));
> +
> +				if (p4d_none(*p4d))
> +					set_p4d(p4d, *p4d_ref);
> +			}
>   
>   			spin_unlock(pgt_lock);
>   		}
> @@ -1024,6 +1026,7 @@ remove_pagetable(unsigned long start, unsigned long end, bool direct)
>   void __ref vmemmap_free(unsigned long start, unsigned long end)
>   {
>   	remove_pagetable(start, end, false);
> +	sync_global_pgds(start, end - 1);

This does fix the HMM crash that I was seeing in hmm-next.

thanks,
John Hubbard
NVIDIA

>   }
>   
>   #ifdef CONFIG_MEMORY_HOTREMOVE
> -- 
> 2.4.11
> 
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