linux-kernel - Re: [PATCH -V2] /proc/PID/smaps: Add PMD migration entry parsing

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Date:   Thu, 2 Apr 2020 08:44:37 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Zi Yan <ziy@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Jérôme Glisse <jglisse@...hat.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>
Subject: Re: [PATCH -V2] /proc/PID/smaps: Add PMD migration entry parsing

On Thu 02-04-20 10:00:31, Huang, Ying wrote:
> From: Huang Ying <ying.huang@...el.com>
> 
> 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.
> 
> Before the patch, for a fully populated 400 MB anonymous VMA, sometimes some THP
> pages under migration may be lost as follows.

Interesting. How did you reproduce this?
[...]

> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 8d382d4ec067..9c72f9ce2dd8 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -546,10 +546,19 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	struct mem_size_stats *mss = walk->private;
>  	struct vm_area_struct *vma = walk->vma;
>  	bool locked = !!(vma->vm_flags & VM_LOCKED);
> -	struct page *page;
> +	struct page *page = NULL;
>  
> -	/* 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(thp_migration_supported() && is_swap_pmd(*pmd))) {
> +		swp_entry_t entry = pmd_to_swp_entry(*pmd);
> +
> +		if (is_migration_entry(entry))
> +			page = migration_entry_to_page(entry);
> +		else
> +			VM_WARN_ON_ONCE(1);

Could you explain why do we need this WARN_ON? I haven't really checked
the swap support for THP but cannot we have normal swap pmd entries?

> +	}
>  	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;
>  	}
> -- 
> 2.25.0

-- 
Michal Hocko
SUSE Labs