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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc:     Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Zi Yan <ziy@nvidia.com>, Vlastimil Babka <vbabka@suse.cz>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Michal Hocko <mhocko@suse.com>,
        =?utf-8?B?SsOp?= =?utf-8?B?csO0bWU=?= Glisse 
        <jglisse@redhat.com>, Yang Shi <yang.shi@linux.alibaba.com>
Subject: Re: [PATCH] /proc/PID/smaps: Add PMD migration entry parsing
References: <20200331085604.1260162-1-ying.huang@intel.com>
        <49386753-5984-f708-4153-e9c6de632439@yandex-team.ru>
        <87mu7whr88.fsf@yhuang-dev.intel.com>
        <a2ee2441-8ac7-84d2-8c7d-e28d80a6c413@yandex-team.ru>
Date:   Wed, 01 Apr 2020 14:20:40 +0800
In-Reply-To: <a2ee2441-8ac7-84d2-8c7d-e28d80a6c413@yandex-team.ru> (Konstantin
        Khlebnikov's message of "Wed, 1 Apr 2020 09:03:45 +0300")
Message-ID: <87a73viv5z.fsf@yhuang-dev.intel.com>
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Konstantin Khlebnikov <khlebnikov@yandex-team.ru> writes:

> On 01/04/2020 05.31, Huang, Ying wrote:
>> Konstantin Khlebnikov <khlebnikov@yandex-team.ru> writes:
>>
>>> On 31/03/2020 11.56, Huang, Ying wrote:
>>>> From: Huang Ying <ying.huang@intel.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@intel.com>
>>>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>>>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>>> Cc: Zi Yan <ziy@nvidia.com>
>>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>>> Cc: Alexey Dobriyan <adobriyan@gmail.com>
>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>> Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>>>> Cc: "Jérôme Glisse" <jglisse@redhat.com>
>>>> Cc: Yang Shi <yang.shi@linux.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;
>>>
>>>          struct page *page = NULL;
>>
>> Looks good.  Will do this in the next version.
>>
>>>>    -	/* 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));
>>>> +		page = migration_entry_to_page(entry);
>>>
>>>                  if (is_migration_entry(entry))
>>>                          page = migration_entry_to_page(entry);
>>>
>>> Seems safer and doesn't add much code.
>>
>> With this, we lose an opportunity to capture some bugs during debugging.
>> Right?
>
> You can keep VM_BUG_ON or VM_WARN_ON_ONCE
>
> Off-by-page in statistics isn't a big deal and not a good reason to crash (even debug) kernel.
> But for normal build should use safe behaviour if this isn't hard.

Sounds reasonable!  Will revise the code.  Thanks!

Best Regards,
Huang, Ying

>>
>> Best Regards,
>> Huang, Ying
>>
>>>> +	} 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;
>>>>    	}
>>>>