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

[Yang Shi]

On 4/2/20 12:44 AM, Michal Hocko wrote:
> On Thu 02-04-20 15:03:23, Huang, Ying wrote:
> > 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.
> 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?