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

[Huang\, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Tue, 31 Mar 2020 16:56:04 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:
>
>> From: Huang Ying <ying.huang@xxxxxxxxx>
>> 
>> 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.
>
> It would be helpful to show the before-and-after output in the changelog.

OK.

>> --- 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;
>>  
>> -	/* 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));
>
> I don't think this justifies nuking the kernel.  A
> WARN()-and-recover would be better.

Yes.  Will change this in the next version.

Best Regards,
Huang, Ying

>> +		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;
>>  	}