Re: [PATCH] mm: memcontrol: Make the walk_page_range() limit obvious

[Michal Hocko <mhocko@kernel.org>]

On Wed 31-08-16 16:04:57, James Morse wrote:
> Trying to walk all of virtual memory requires architecture specific
> knowledge. On x86_64, addresses must be sign extended from bit 48,
> whereas on arm64 the top VA_BITS of address space have their own set
> of page tables.
> 
> mem_cgroup_count_precharge() and mem_cgroup_move_charge() both call
> walk_page_range() on the range 0 to ~0UL, neither provide a pte_hole
> callback, which causes the current implementation to skip non-vma regions.
> 
> As this call only expects to walk user address space, make it walk
> 0 to  'highest_vm_end'.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
> This is in preparation for a RFC series that allows walk_page_range() to
> walk kernel page tables too.

OK, so do I get it right that this is only needed with that change?
Because AFAICS walk_page_range will be bound to the last vma->vm_end
right now. If this is the case this should be mentioned in the changelog
because the above might confuse somebody to think this is a bug fix.

Other than that this seams reasonable to me.

> 
>  mm/memcontrol.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2ff0289ad061..bfd54b43beb9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4712,7 +4712,8 @@ static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm)
>  		.mm = mm,
>  	};
>  	down_read(&mm->mmap_sem);
> -	walk_page_range(0, ~0UL, &mem_cgroup_count_precharge_walk);
> +	walk_page_range(0, mm->highest_vm_end,
> +			&mem_cgroup_count_precharge_walk);
>  	up_read(&mm->mmap_sem);
>  
>  	precharge = mc.precharge;
> @@ -5000,7 +5001,8 @@ retry:
>  	 * When we have consumed all precharges and failed in doing
>  	 * additional charge, the page walk just aborts.
>  	 */
> -	walk_page_range(0, ~0UL, &mem_cgroup_move_charge_walk);
> +	walk_page_range(0, mc.mm->highest_vm_end, &mem_cgroup_move_charge_walk);
> +
>  	up_read(&mc.mm->mmap_sem);
>  	atomic_dec(&mc.from->moving_account);
>  }
> -- 
> 2.8.0.rc3
> 
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-- 
Michal Hocko
SUSE Labs

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