Re: [PATCH v1 1/4] mm: introduce MADV_COLD

[Minchan Kim <minchan@kernel.org>]

Hi Joel,

On Tue, Jun 04, 2019 at 04:38:41PM -0400, Joel Fernandes wrote:
> On Mon, Jun 03, 2019 at 02:36:52PM +0900, Minchan Kim wrote:
> > When a process expects no accesses to a certain memory range, it could
> > give a hint to kernel that the pages can be reclaimed when memory pressure
> > happens but data should be preserved for future use.  This could reduce
> > workingset eviction so it ends up increasing performance.
> > 
> > This patch introduces the new MADV_COLD hint to madvise(2) syscall.
> > MADV_COLD can be used by a process to mark a memory range as not expected
> > to be used in the near future. The hint can help kernel in deciding which
> > pages to evict early during memory pressure.
> > 
> > It works for every LRU pages like MADV_[DONTNEED|FREE]. IOW, It moves
> > 
> > 	active file page -> inactive file LRU
> > 	active anon page -> inacdtive anon LRU
> > 
> > Unlike MADV_FREE, it doesn't move active anonymous pages to inactive
> > files's head because MADV_COLD is a little bit different symantic.
> > MADV_FREE means it's okay to discard when the memory pressure because
> > the content of the page is *garbage* so freeing such pages is almost zero
> > overhead since we don't need to swap out and access afterward causes just
> > minor fault. Thus, it would make sense to put those freeable pages in
> > inactive file LRU to compete other used-once pages. Even, it could
> > give a bonus to make them be reclaimed on swapless system. However,
> > MADV_COLD doesn't mean garbage so reclaiming them requires swap-out/in
> > in the end. So it's better to move inactive anon's LRU list, not file LRU.
> > Furthermore, it would help to avoid unnecessary scanning of cold anonymous
> > if system doesn't have a swap device.
> > 
> > All of error rule is same with MADV_DONTNEED.
> > 
> > Note:
> > This hint works with only private pages(IOW, page_mapcount(page) < 2)
> > because shared page could have more chance to be accessed from other
> > processes sharing the page although the caller reset the reference bits.
> > It ends up preventing the reclaim of the page and wastes CPU cycle.
> > 
> > * RFCv2
> >  * add more description - mhocko
> > 
> > * RFCv1
> >  * renaming from MADV_COOL to MADV_COLD - hannes
> > 
> > * internal review
> >  * use clear_page_youn in deactivate_page - joelaf
> >  * Revise the description - surenb
> >  * Renaming from MADV_WARM to MADV_COOL - surenb
> > 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  include/linux/page-flags.h             |   1 +
> >  include/linux/page_idle.h              |  15 ++++
> >  include/linux/swap.h                   |   1 +
> >  include/uapi/asm-generic/mman-common.h |   1 +
> >  mm/internal.h                          |   2 +-
> >  mm/madvise.c                           | 115 ++++++++++++++++++++++++-
> >  mm/oom_kill.c                          |   2 +-
> >  mm/swap.c                              |  43 +++++++++
> >  8 files changed, 176 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 9f8712a4b1a5..58b06654c8dd 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -424,6 +424,7 @@ static inline bool set_hwpoison_free_buddy_page(struct page *page)
> >  TESTPAGEFLAG(Young, young, PF_ANY)
> >  SETPAGEFLAG(Young, young, PF_ANY)
> >  TESTCLEARFLAG(Young, young, PF_ANY)
> > +CLEARPAGEFLAG(Young, young, PF_ANY)
> >  PAGEFLAG(Idle, idle, PF_ANY)
> >  #endif
> >  
> > diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
> > index 1e894d34bdce..f3f43b317150 100644
> > --- a/include/linux/page_idle.h
> > +++ b/include/linux/page_idle.h
> > @@ -19,6 +19,11 @@ static inline void set_page_young(struct page *page)
> >  	SetPageYoung(page);
> >  }
> >  
> > +static inline void clear_page_young(struct page *page)
> > +{
> > +	ClearPageYoung(page);
> > +}
> > +
> >  static inline bool test_and_clear_page_young(struct page *page)
> >  {
> >  	return TestClearPageYoung(page);
> > @@ -65,6 +70,16 @@ static inline void set_page_young(struct page *page)
> >  	set_bit(PAGE_EXT_YOUNG, &page_ext->flags);
> >  }
> >  
> > +static void clear_page_young(struct page *page)
> > +{
> > +	struct page_ext *page_ext = lookup_page_ext(page);
> > +
> > +	if (unlikely(!page_ext))
> > +		return;
> > +
> > +	clear_bit(PAGE_EXT_YOUNG, &page_ext->flags);
> > +}
> > +
> >  static inline bool test_and_clear_page_young(struct page *page)
> >  {
> >  	struct page_ext *page_ext = lookup_page_ext(page);
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index de2c67a33b7e..0ce997edb8bb 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -340,6 +340,7 @@ extern void lru_add_drain_cpu(int cpu);
> >  extern void lru_add_drain_all(void);
> >  extern void rotate_reclaimable_page(struct page *page);
> >  extern void deactivate_file_page(struct page *page);
> > +extern void deactivate_page(struct page *page);
> >  extern void mark_page_lazyfree(struct page *page);
> >  extern void swap_setup(void);
> >  
> > diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> > index bea0278f65ab..1190f4e7f7b9 100644
> > --- a/include/uapi/asm-generic/mman-common.h
> > +++ b/include/uapi/asm-generic/mman-common.h
> > @@ -43,6 +43,7 @@
> >  #define MADV_SEQUENTIAL	2		/* expect sequential page references */
> >  #define MADV_WILLNEED	3		/* will need these pages */
> >  #define MADV_DONTNEED	4		/* don't need these pages */
> > +#define MADV_COLD	5		/* deactivatie these pages */
> >  
> >  /* common parameters: try to keep these consistent across architectures */
> >  #define MADV_FREE	8		/* free pages only if memory pressure */
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 9eeaf2b95166..75a4f96ec0fb 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -43,7 +43,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf);
> >  void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
> >  		unsigned long floor, unsigned long ceiling);
> >  
> > -static inline bool can_madv_dontneed_vma(struct vm_area_struct *vma)
> > +static inline bool can_madv_lru_vma(struct vm_area_struct *vma)
> >  {
> >  	return !(vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP));
> >  }
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 628022e674a7..ab158766858a 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -40,6 +40,7 @@ static int madvise_need_mmap_write(int behavior)
> >  	case MADV_REMOVE:
> >  	case MADV_WILLNEED:
> >  	case MADV_DONTNEED:
> > +	case MADV_COLD:
> >  	case MADV_FREE:
> >  		return 0;
> >  	default:
> > @@ -307,6 +308,113 @@ static long madvise_willneed(struct vm_area_struct *vma,
> >  	return 0;
> >  }
> >  
> > +static int madvise_cold_pte_range(pmd_t *pmd, unsigned long addr,
> > +				unsigned long end, struct mm_walk *walk)
> > +{
> > +	pte_t *orig_pte, *pte, ptent;
> > +	spinlock_t *ptl;
> > +	struct page *page;
> > +	struct vm_area_struct *vma = walk->vma;
> > +	unsigned long next;
> > +
> > +	next = pmd_addr_end(addr, end);
> > +	if (pmd_trans_huge(*pmd)) {
> > +		ptl = pmd_trans_huge_lock(pmd, vma);
> > +		if (!ptl)
> > +			return 0;
> > +
> > +		if (is_huge_zero_pmd(*pmd))
> > +			goto huge_unlock;
> > +
> > +		page = pmd_page(*pmd);
> > +		if (page_mapcount(page) > 1)
> > +			goto huge_unlock;
> > +
> > +		if (next - addr != HPAGE_PMD_SIZE) {
> > +			int err;
> > +
> > +			get_page(page);
> > +			spin_unlock(ptl);
> > +			lock_page(page);
> > +			err = split_huge_page(page);
> > +			unlock_page(page);
> > +			put_page(page);
> > +			if (!err)
> > +				goto regular_page;
> > +			return 0;
> > +		}
> > +
> > +		pmdp_test_and_clear_young(vma, addr, pmd);
> > +		deactivate_page(page);
> > +huge_unlock:
> > +		spin_unlock(ptl);
> > +		return 0;
> > +	}
> > +
> > +	if (pmd_trans_unstable(pmd))
> > +		return 0;
> > +
> > +regular_page:
> > +	orig_pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> > +	for (pte = orig_pte; addr < end; pte++, addr += PAGE_SIZE) {
> > +		ptent = *pte;
> > +
> > +		if (pte_none(ptent))
> > +			continue;
> > +
> > +		if (!pte_present(ptent))
> > +			continue;
> > +
> > +		page = vm_normal_page(vma, addr, ptent);
> > +		if (!page)
> > +			continue;
> > +
> > +		if (page_mapcount(page) > 1)
> > +			continue;
> > +
> > +		ptep_test_and_clear_young(vma, addr, pte);
> 
> Wondering here how it interacts with idle page tracking. Here since young
> flag is cleared by the cold hint, page_referenced_one() or
> page_idle_clear_pte_refs_one() will not be able to clear the page-idle flag
> if it was previously set since it does not know any more that a page was
> actively referenced.

ptep_test_and_clear_young doesn't change PG_idle/young so idle page tracking
doesn't affect.

> 
> Should you also be clearing the page-idle flag here if the ptep young/accessed
> bit was previously set, just so that page-idle tracking works smoothly when
> this hint is concurrently applied?

deactivate_page will remove PG_young bit so that the page will be reclaimed.
Do I miss your point?

> 
> > +		deactivate_page(page);
> > +	}
> > +
> > +	pte_unmap_unlock(orig_pte, ptl);
> > +	cond_resched();
> > +
> > +	return 0;
> > +}
> > +
> > +static void madvise_cold_page_range(struct mmu_gather *tlb,
> > +			     struct vm_area_struct *vma,
> > +			     unsigned long addr, unsigned long end)
> > +{
> > +	struct mm_walk cool_walk = {
> > +		.pmd_entry = madvise_cold_pte_range,
> > +		.mm = vma->vm_mm,
> > +	};
> > +
> > +	tlb_start_vma(tlb, vma);
> > +	walk_page_range(addr, end, &cool_walk);
> > +	tlb_end_vma(tlb, vma);
> > +}
> > +
> > +static long madvise_cold(struct vm_area_struct *vma,
> > +			struct vm_area_struct **prev,
> > +			unsigned long start_addr, unsigned long end_addr)
> > +{
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	struct mmu_gather tlb;
> > +
> > +	*prev = vma;
> > +	if (!can_madv_lru_vma(vma))
> > +		return -EINVAL;
> > +
> > +	lru_add_drain();
> > +	tlb_gather_mmu(&tlb, mm, start_addr, end_addr);
> > +	madvise_cold_page_range(&tlb, vma, start_addr, end_addr);
> > +	tlb_finish_mmu(&tlb, start_addr, end_addr);
> > +
> > +	return 0;
> > +}
> > +
> >  static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >  				unsigned long end, struct mm_walk *walk)
> >  
> > @@ -519,7 +627,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
> >  				  int behavior)
> >  {
> >  	*prev = vma;
> > -	if (!can_madv_dontneed_vma(vma))
> > +	if (!can_madv_lru_vma(vma))
> >  		return -EINVAL;
> >  
> >  	if (!userfaultfd_remove(vma, start, end)) {
> > @@ -541,7 +649,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
> >  			 */
> >  			return -ENOMEM;
> >  		}
> > -		if (!can_madv_dontneed_vma(vma))
> > +		if (!can_madv_lru_vma(vma))
> >  			return -EINVAL;
> >  		if (end > vma->vm_end) {
> >  			/*
> > @@ -695,6 +803,8 @@ madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
> >  		return madvise_remove(vma, prev, start, end);
> >  	case MADV_WILLNEED:
> >  		return madvise_willneed(vma, prev, start, end);
> > +	case MADV_COLD:
> > +		return madvise_cold(vma, prev, start, end);
> >  	case MADV_FREE:
> >  	case MADV_DONTNEED:
> >  		return madvise_dontneed_free(vma, prev, start, end, behavior);
> > @@ -716,6 +826,7 @@ madvise_behavior_valid(int behavior)
> >  	case MADV_WILLNEED:
> >  	case MADV_DONTNEED:
> >  	case MADV_FREE:
> > +	case MADV_COLD:
> >  #ifdef CONFIG_KSM
> >  	case MADV_MERGEABLE:
> >  	case MADV_UNMERGEABLE:
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 5a58778c91d4..f73d5f5145f0 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -515,7 +515,7 @@ bool __oom_reap_task_mm(struct mm_struct *mm)
> >  	set_bit(MMF_UNSTABLE, &mm->flags);
> >  
> >  	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
> > -		if (!can_madv_dontneed_vma(vma))
> > +		if (!can_madv_lru_vma(vma))
> >  			continue;
> >  
> >  		/*
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 7b079976cbec..cebedab15aa2 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -47,6 +47,7 @@ int page_cluster;
> >  static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);
> >  static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);
> >  static DEFINE_PER_CPU(struct pagevec, lru_deactivate_file_pvecs);
> > +static DEFINE_PER_CPU(struct pagevec, lru_deactivate_pvecs);
> >  static DEFINE_PER_CPU(struct pagevec, lru_lazyfree_pvecs);
> >  #ifdef CONFIG_SMP
> >  static DEFINE_PER_CPU(struct pagevec, activate_page_pvecs);
> > @@ -538,6 +539,23 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
> >  	update_page_reclaim_stat(lruvec, file, 0);
> >  }
> >  
> > +static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec,
> > +			    void *arg)
> > +{
> > +	if (PageLRU(page) && PageActive(page) && !PageUnevictable(page)) {
> > +		int file = page_is_file_cache(page);
> > +		int lru = page_lru_base_type(page);
> > +
> > +		del_page_from_lru_list(page, lruvec, lru + LRU_ACTIVE);
> > +		ClearPageActive(page);
> > +		ClearPageReferenced(page);
> > +		clear_page_young(page);
> 
> I was curious, how does the above interact with clear_refs?

clear_refs couldn't find the access any longer. However, it's the
intention because we couldn't reclaim the page without it.

> 
> It appears that a range that is marked as cold will appear to be unreferenced
> (since referenced flag is cleared) even though it may have been referenced in
> the past. Very least, I believe this behavior should be documented somewhere.

Okay, I will add some comment.

Thanks.