Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared pages

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Aug 23, 2018 at 09:30:35AM +0200, Michal Hocko wrote:
> On Wed 22-08-18 09:48:16, Mike Kravetz wrote:
> > On 08/22/2018 05:28 AM, Michal Hocko wrote:
> > > On Tue 21-08-18 18:10:42, Mike Kravetz wrote:
> > > [...]
> > >> diff --git a/mm/rmap.c b/mm/rmap.c
> > >> index eb477809a5c0..8cf853a4b093 100644
> > >> --- a/mm/rmap.c
> > >> +++ b/mm/rmap.c
> > >> @@ -1362,11 +1362,21 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> > >>  	}
> > >>  
> > >>  	/*
> > >> -	 * We have to assume the worse case ie pmd for invalidation. Note that
> > >> -	 * the page can not be free in this function as call of try_to_unmap()
> > >> -	 * must hold a reference on the page.
> > >> +	 * For THP, we have to assume the worse case ie pmd for invalidation.
> > >> +	 * For hugetlb, it could be much worse if we need to do pud
> > >> +	 * invalidation in the case of pmd sharing.
> > >> +	 *
> > >> +	 * Note that the page can not be free in this function as call of
> > >> +	 * try_to_unmap() must hold a reference on the page.
> > >>  	 */
> > >>  	end = min(vma->vm_end, start + (PAGE_SIZE << compound_order(page)));
> > >> +	if (PageHuge(page)) {
> > >> +		/*
> > >> +		 * If sharing is possible, start and end will be adjusted
> > >> +		 * accordingly.
> > >> +		 */
> > >> +		(void)huge_pmd_sharing_possible(vma, &start, &end);
> > >> +	}
> > >>  	mmu_notifier_invalidate_range_start(vma->vm_mm, start, end);
> > > 
> > > I do not get this part. Why don't we simply unconditionally invalidate
> > > the whole huge page range?
> > 
> > In this routine, we are only unmapping a single page.  The existing code
> > is limiting the invalidate range to that page size: 4K or 2M.  With shared
> > PMDs, we have the possibility of unmapping a PUD_SIZE area: 1G.  I don't
> > think we want to unconditionally invalidate 1G.  Is that what you are asking?
> 
> But we know that huge_pmd_unshare unmapped a shared pte so we know when
> to flush 2MB or 1GB. I really do not like how huge_pmd_sharing_possible
> a) duplicates some checks and b) it updates start/stop out of line.

My reading on this is that mmu_notifier_invalidate_range_start() has to be
called from sleepable context on the full range that *can* be invalidated
before following mmu_notifier_invalidate_range_end().

In this case huge_pmd_unshare() may unmap aligned PUD_SIZE around the PMD
page that effectively enlarge range that has to be covered by
mmu_notifier_invalidate_range_start(). We cannot yet know if there's any
shared page tables in the range, so we need to go with worst case
scenario.

I don't see conceptually better solution than what is proposed.

-- 
 Kirill A. Shutemov