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

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Date:   Tue, 31 Mar 2020 08:24:07 -0400
From:   Zi Yan <ziy@...dia.com>
To:     "Huang, Ying" <ying.huang@...el.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Michal Hocko <mhocko@...e.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Jérôme Glisse <jglisse@...hat.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>
Subject: Re: [PATCH] /proc/PID/smaps: Add PMD migration entry parsing

On 31 Mar 2020, at 4:56, 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.
>
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Alexey Dobriyan <adobriyan@...il.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Cc: "Jérôme Glisse" <jglisse@...hat.com>
> Cc: Yang Shi <yang.shi@...ux.alibaba.com>
> ---
>  fs/proc/task_mmu.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 8d382d4ec067..b5b3aef8cb3b 100644
> --- 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;

Like Konstantin pointed out in another email, you could initialize page to NULL here.
Plus you do not need the “else-return” below, if you do that.

>
> -       /* 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))) {

Should be:
          } else if (unlikely(thp_migration_support() && is_swap_pmd(*pmd))) {

Otherwise, when THP migration is disabled and the PMD is under splitting, VM_BUG_ON
will be triggered.

> +               swp_entry_t entry = pmd_to_swp_entry(*pmd);
> +
> +               VM_BUG_ON(!is_migration_entry(entry));
> +               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;
>         }
> --
> 2.25.0

Everything else looks good to me. Thanks.

With the fixes mentioned above, you can add
Reviewed-by: Zi Yan <ziy@...dia.com>


—
Best Regards,
Yan Zi

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