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

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Message-ID: <C497B0EF-1B3A-4E4E-8888-D74B2CEFE2B3@nvidia.com>
Date:   Tue, 31 Mar 2020 22:42:06 -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 22:24, Huang, Ying wrote:

> External email: Use caution opening links or attachments
>
>
> Zi Yan <ziy@...dia.com> writes:
>
>> 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.
>
> Yes.  That looks better.  Will change this in the next version.

Thanks.

>
>>>
>>> -       /* 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.
>
> We hold the PMD page table lock when call smaps_pmd_entry().  How does
> PMD splitting trigger VM_BUG_ON()?

Oh, I missed that. Your original code is right. Thank you for the explanation.



—
Best Regards,
Yan Zi

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