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

[Joel Fernandes <joel@joelfernandes.org>]

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.

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(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?

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.

I believe it is best for us to make the clear_refs technique behave similar
to the idle-page tracking for the purposes of figuring out which pages are
referenced, so that clear_refs is more immune to this hint.

thanks,

 - Joel