Re: [PATCH -V2] /proc/PID/smaps: Add PMD migration entry parsing

["Huang\, Ying" <ying.huang@intel.com>]

Michal Hocko <mhocko@kernel.org> writes:

> On Thu 02-04-20 10:00:31, 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.
>> 
>> Before the patch, for a fully populated 400 MB anonymous VMA, sometimes some THP
>> pages under migration may be lost as follows.
>
> Interesting. How did you reproduce this?
> [...]

I run the pmbench in background to eat memory, then run
`/usr/bin/migratepages` and `cat /proc/PID/smaps` every second.  The
issue can be reproduced within 60 seconds.

>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index 8d382d4ec067..9c72f9ce2dd8 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -546,10 +546,19 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>>  	struct mem_size_stats *mss = walk->private;
>>  	struct vm_area_struct *vma = walk->vma;
>>  	bool locked = !!(vma->vm_flags & VM_LOCKED);
>> -	struct page *page;
>> +	struct page *page = NULL;
>>  
>> -	/* 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(thp_migration_supported() && is_swap_pmd(*pmd))) {
>> +		swp_entry_t entry = pmd_to_swp_entry(*pmd);
>> +
>> +		if (is_migration_entry(entry))
>> +			page = migration_entry_to_page(entry);
>> +		else
>> +			VM_WARN_ON_ONCE(1);
>
> Could you explain why do we need this WARN_ON? I haven't really checked
> the swap support for THP but cannot we have normal swap pmd entries?

I have some patches to add the swap pmd entry support, but they haven't
been merged yet.

Similar checks are for all THP migration code paths, so I follow the
same style.

Best Regards,
Huang, Ying