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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>
Date:   Wed, 01 Apr 2020 10:31:03 +0800
In-Reply-To: <49386753-5984-f708-4153-e9c6de632439@yandex-team.ru> (Konstantin
        Khlebnikov's message of "Tue, 31 Mar 2020 12:51:38 +0300")
Message-ID: <87mu7whr88.fsf@yhuang-dev.intel.com>
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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?

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;
>>   	}
>>