Michal Hocko <mhocko@xxxxxxxxxx> writes:
> On Thu 02-04-20 10:00:31, Huang, Ying 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.
>>
>> 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
