Re: [PATCHv2 6/8] khugepaged: Allow to collapse PTE-mapped compound pages

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

On Wed, Apr 08, 2020 at 11:57:27AM -0700, Yang Shi wrote:
> 
> 
> On 4/8/20 6:29 AM, Kirill A. Shutemov wrote:
> > On Mon, Apr 06, 2020 at 02:29:35PM -0700, Yang Shi wrote:
> > > 
> > > On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
> > > > We can collapse PTE-mapped compound pages. We only need to avoid
> > > > handling them more than once: lock/unlock page only once if it's present
> > > > in the PMD range multiple times as it handled on compound level. The
> > > > same goes for LRU isolation and putback.
> > > > 
> > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > > ---
> > > >    mm/khugepaged.c | 103 ++++++++++++++++++++++++++++++++----------------
> > > >    1 file changed, 68 insertions(+), 35 deletions(-)
> > > > 
> > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > > index 1e7e6543ebca..49e56e4e30d1 100644
> > > > --- a/mm/khugepaged.c
> > > > +++ b/mm/khugepaged.c
> > > > @@ -515,23 +515,37 @@ void __khugepaged_exit(struct mm_struct *mm)
> > > >    static void release_pte_page(struct page *page)
> > > >    {
> > > > -	dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
> > > > +	mod_node_page_state(page_pgdat(page),
> > > > +			NR_ISOLATED_ANON + page_is_file_cache(page),
> > > > +			-compound_nr(page));
> > > >    	unlock_page(page);
> > > >    	putback_lru_page(page);
> > > >    }
> > > > -static void release_pte_pages(pte_t *pte, pte_t *_pte)
> > > > +static void release_pte_pages(pte_t *pte, pte_t *_pte,
> > > > +		struct list_head *compound_pagelist)
> > > >    {
> > > > +	struct page *page, *tmp;
> > > > +
> > > >    	while (--_pte >= pte) {
> > > >    		pte_t pteval = *_pte;
> > > > -		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)))
> > > > -			release_pte_page(pte_page(pteval));
> > > > +
> > > > +		page = pte_page(pteval);
> > > > +		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)) &&
> > > > +				!PageCompound(page))
> > > > +			release_pte_page(page);
> > > > +	}
> > > > +
> > > > +	list_for_each_entry_safe(page, tmp, compound_pagelist, lru) {
> > > > +		list_del(&page->lru);
> > > > +		release_pte_page(page);
> > > >    	}
> > > >    }
> > > >    static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > > >    					unsigned long address,
> > > > -					pte_t *pte)
> > > > +					pte_t *pte,
> > > > +					struct list_head *compound_pagelist)
> > > >    {
> > > >    	struct page *page = NULL;
> > > >    	pte_t *_pte;
> > > > @@ -561,13 +575,21 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > > >    			goto out;
> > > >    		}
> > > > -		/* TODO: teach khugepaged to collapse THP mapped with pte */
> > > > +		VM_BUG_ON_PAGE(!PageAnon(page), page);
> > > > +
> > > >    		if (PageCompound(page)) {
> > > > -			result = SCAN_PAGE_COMPOUND;
> > > > -			goto out;
> > > > -		}
> > > > +			struct page *p;
> > > > +			page = compound_head(page);
> > > > -		VM_BUG_ON_PAGE(!PageAnon(page), page);
> > > > +			/*
> > > > +			 * Check if we have dealt with the compound page
> > > > +			 * already
> > > > +			 */
> > > > +			list_for_each_entry(p, compound_pagelist, lru) {
> > > > +				if (page == p)
> > > > +					goto next;
> > > > +			}
> > > > +		}
> > > >    		/*
> > > >    		 * We can do it before isolate_lru_page because the
> > > > @@ -597,19 +619,15 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > > >    			result = SCAN_PAGE_COUNT;
> > > >    			goto out;
> > > >    		}
> > > > -		if (pte_write(pteval)) {
> > > > -			writable = true;
> > > > -		} else {
> > > > -			if (PageSwapCache(page) &&
> > > > -			    !reuse_swap_page(page, NULL)) {
> > > > -				unlock_page(page);
> > > > -				result = SCAN_SWAP_CACHE_PAGE;
> > > > -				goto out;
> > > > -			}
> > > > +		if (!pte_write(pteval) && PageSwapCache(page) &&
> > > > +				!reuse_swap_page(page, NULL)) {
> > > >    			/*
> > > > -			 * Page is not in the swap cache. It can be collapsed
> > > > -			 * into a THP.
> > > > +			 * Page is in the swap cache and cannot be re-used.
> > > > +			 * It cannot be collapsed into a THP.
> > > >    			 */
> > > > +			unlock_page(page);
> > > > +			result = SCAN_SWAP_CACHE_PAGE;
> > > > +			goto out;
> > > >    		}
> > > >    		/*
> > > > @@ -621,16 +639,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > > >    			result = SCAN_DEL_PAGE_LRU;
> > > >    			goto out;
> > > >    		}
> > > > -		inc_node_page_state(page,
> > > > -				NR_ISOLATED_ANON + page_is_file_cache(page));
> > > > +		mod_node_page_state(page_pgdat(page),
> > > > +				NR_ISOLATED_ANON + page_is_file_cache(page),
> > > > +				compound_nr(page));
> > > >    		VM_BUG_ON_PAGE(!PageLocked(page), page);
> > > >    		VM_BUG_ON_PAGE(PageLRU(page), page);
> > > > +		if (PageCompound(page))
> > > > +			list_add_tail(&page->lru, compound_pagelist);
> > > > +next:
> > > >    		/* There should be enough young pte to collapse the page */
> > > >    		if (pte_young(pteval) ||
> > > >    		    page_is_young(page) || PageReferenced(page) ||
> > > >    		    mmu_notifier_test_young(vma->vm_mm, address))
> > > >    			referenced++;
> > > > +
> > > > +		if (pte_write(pteval))
> > > > +			writable = true;
> > > >    	}
> > > >    	if (likely(writable)) {
> > > >    		if (likely(referenced)) {
> > > > @@ -644,7 +669,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > > >    	}
> > > >    out:
> > > > -	release_pte_pages(pte, _pte);
> > > > +	release_pte_pages(pte, _pte, compound_pagelist);
> > > >    	trace_mm_collapse_huge_page_isolate(page, none_or_zero,
> > > >    					    referenced, writable, result);
> > > >    	return 0;
> > > > @@ -653,13 +678,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > > >    static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> > > >    				      struct vm_area_struct *vma,
> > > >    				      unsigned long address,
> > > > -				      spinlock_t *ptl)
> > > > +				      spinlock_t *ptl,
> > > > +				      struct list_head *compound_pagelist)
> > > >    {
> > > > +	struct page *src_page, *tmp;
> > > >    	pte_t *_pte;
> > > >    	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > > >    				_pte++, page++, address += PAGE_SIZE) {
> > > >    		pte_t pteval = *_pte;
> > > > -		struct page *src_page;
> > > >    		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > > >    			clear_user_highpage(page, address);
> > > > @@ -679,7 +705,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> > > >    		} else {
> > > >    			src_page = pte_page(pteval);
> > > >    			copy_user_highpage(page, src_page, address, vma);
> > > > -			release_pte_page(src_page);
> > > >    			/*
> > > >    			 * ptl mostly unnecessary, but preempt has to
> > > >    			 * be disabled to update the per-cpu stats
> > > > @@ -693,9 +718,18 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> > > >    			pte_clear(vma->vm_mm, address, _pte);
> > > >    			page_remove_rmap(src_page, false);
> > > >    			spin_unlock(ptl);
> > > > -			free_page_and_swap_cache(src_page);
> > > > +			if (!PageCompound(src_page)) {
> > > > +				release_pte_page(src_page);
> > > > +				free_page_and_swap_cache(src_page);
> > > > +			}
> > > >    		}
> > > >    	}
> > > > +
> > > > +	list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
> > > > +		list_del(&src_page->lru);
> > > > +		release_pte_page(src_page);
> > > > +		free_page_and_swap_cache(src_page);
> > > > +	}
> > > It looks this may mess up the PTE-mapped THP's refcount if it has multiple
> > > PTE-mapped subpages since put_page() is not called for every PTE-mapped
> > > subpages.
> > Good catch!
> > 
> > I *think* something like this should fix the issue (untested):
> 
> Is this an incremental fix?

Yes. I will fold it in before the next submission.

> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index bfb6155f1d69..9a96f9bff798 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -570,6 +570,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
> >   	list_for_each_entry_safe(page, tmp, compound_pagelist, lru) {
> >   		list_del(&page->lru);
> >   		release_pte_page(page);
> > +		put_page(page);
> >   	}
> >   }
> > @@ -682,8 +683,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >   		VM_BUG_ON_PAGE(!PageLocked(page), page);
> >   		VM_BUG_ON_PAGE(PageLRU(page), page);
> > -		if (PageCompound(page))
> > +		if (PageCompound(page)) {
> >   			list_add_tail(&page->lru, compound_pagelist);
> > +			get_page(page);
> 
> I don't get why we have to bump refcount here? The THP won't go away since
> it is pinned by isolation. I'm supposed calling
> free_page_and_swap_cache(src_page) in each loop looks good enough to dec
> refcount for each PTE-mapped subpage.
> 
> Then calling release_pte_page() for each entry on compund_pagelist outside
> the loop to release the pin from isolation. The compound page would get
> freed via pagevec finally.

Fair enough.

-- 
 Kirill A. Shutemov