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 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.

Sure.  Will add that in the next version.

>> >> 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?

Yes. smaps itself is perfectly fine to ignore it.  I think this is used
to find bugs in other code paths such as THP migration related.

Best Regards,
Huang, Ying