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

[Yang Shi <yang.shi@linux.alibaba.com>]

On 4/2/20 12:44 AM, Michal Hocko wrote:
> On Thu 02-04-20 15:03:23, Huang, Ying wrote:
>> 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.
> Please add that information to the changelog. I was probably too
> optimistic about the migration duration because I found it highly
> unlikely to be visible. I was clearly wrong here.

I believe that depends on the page is shared by how many processes. If 
it is not shared then it should just take dozens micro seconds in my 
test FYI.

>
>>>> 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.
> I haven't checked other migration code paths but what is the reason to
> add the warning here? Even if this shouldn't happen, smaps is perfectly
> fine to ignore that situation, no?