Re: [PATCH] /proc/PID/smaps: Add PMD migration entry parsing

[Andrew Morton]

On Tue, 31 Mar 2020 16:56:04 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:

> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> Now, when read /proc/PID/smaps, the PMD migration entry in page table is simply
> ignored.  To improve the accuracy of /proc/PID/smaps, its parsing and processing
> is added.

It would be helpful to show the before-and-after output in the changelog.

> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -548,8 +548,17 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	bool locked = !!(vma->vm_flags & VM_LOCKED);
>  	struct page *page;
>  
> -	/* FOLL_DUMP will return -EFAULT on huge zero page */
> -	page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> +	if (pmd_present(*pmd)) {
> +		/* FOLL_DUMP will return -EFAULT on huge zero page */
> +		page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> +	} else if (unlikely(is_swap_pmd(*pmd))) {
> +		swp_entry_t entry = pmd_to_swp_entry(*pmd);
> +
> +		VM_BUG_ON(!is_migration_entry(entry));

I don't think this justifies nuking the kernel.  A
WARN()-and-recover would be better.

> +		page = migration_entry_to_page(entry);
> +	} else {
> +		return;
> +	}
>  	if (IS_ERR_OR_NULL(page))
>  		return;
>  	if (PageAnon(page))
> @@ -578,8 +587,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  
>  	ptl = pmd_trans_huge_lock(pmd, vma);
>  	if (ptl) {
> -		if (pmd_present(*pmd))
> -			smaps_pmd_entry(pmd, addr, walk);
> +		smaps_pmd_entry(pmd, addr, walk);
>  		spin_unlock(ptl);
>  		goto out;
>  	}