Re: [PATCH v11 1/7] mm: support madvise(MADV_FREE)

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

On Tue, Jul 08, 2014 at 03:03:38PM +0900, Minchan Kim wrote:
> Linux doesn't have an ability to free pages lazy while other OS
> already have been supported that named by madvise(MADV_FREE).
> 
> The gain is clear that kernel can discard freed pages rather than
> swapping out or OOM if memory pressure happens.
> 
> Without memory pressure, freed pages would be reused by userspace
> without another additional overhead(ex, page fault + allocation
> + zeroing).
> 
> How to work is following as.
> 
> When madvise syscall is called, VM clears dirty bit of ptes of
> the range. If memory pressure happens, VM checks dirty bit of
> page table and if it found still "clean", it means it's a
> "lazyfree pages" so VM could discard the page instead of swapping out.
> Once there was store operation for the page before VM peek a page
> to reclaim, dirty bit is set so VM can swap out the page instead of
> discarding.
> 
> Firstly, heavy users would be general allocators(ex, jemalloc,
> tcmalloc and hope glibc supports it) and jemalloc/tcmalloc already
> have supported the feature for other OS(ex, FreeBSD)
> 
> barrios@blaptop:~/benchmark/ebizzy$ lscpu
> Architecture:          x86_64
> CPU op-mode(s):        32-bit, 64-bit
> Byte Order:            Little Endian
> CPU(s):                4
> On-line CPU(s) list:   0-3
> Thread(s) per core:    2
> Core(s) per socket:    2
> Socket(s):             1
> NUMA node(s):          1
> Vendor ID:             GenuineIntel
> CPU family:            6
> Model:                 42
> Stepping:              7
> CPU MHz:               2801.000
> BogoMIPS:              5581.64
> Virtualization:        VT-x
> L1d cache:             32K
> L1i cache:             32K
> L2 cache:              256K
> L3 cache:              4096K
> NUMA node0 CPU(s):     0-3
> 
> ebizzy benchmark(./ebizzy -S 10 -n 512)
> 
>  vanilla-jemalloc		MADV_free-jemalloc
> 
> 1 thread
> records:  10              records:  10
> avg:      7682.10         avg:      15306.10
> std:      62.35(0.81%)    std:      347.99(2.27%)
> max:      7770.00         max:      15622.00
> min:      7598.00         min:      14772.00
> 
> 2 thread
> records:  10              records:  10
> avg:      12747.50        avg:      24171.00
> std:      792.06(6.21%)   std:      895.18(3.70%)
> max:      13337.00        max:      26023.00
> min:      10535.00        min:      23152.00
> 
> 4 thread
> records:  10              records:  10
> avg:      16474.60        avg:      33717.90
> std:      1496.45(9.08%)  std:      2008.97(5.96%)
> max:      17877.00        max:      35958.00
> min:      12224.00        min:      29565.00
> 
> 8 thread
> records:  10              records:  10
> avg:      16778.50        avg:      33308.10
> std:      825.53(4.92%)   std:      1668.30(5.01%)
> max:      17543.00        max:      36010.00
> min:      14576.00        min:      29577.00
> 
> 16 thread
> records:  10              records:  10
> avg:      20614.40        avg:      35516.30
> std:      602.95(2.92%)   std:      1283.65(3.61%)
> max:      21753.00        max:      37178.00
> min:      19605.00        min:      33217.00
> 
> 32 thread
> records:  10              records:  10
> avg:      22771.70        avg:      36018.50
> std:      598.94(2.63%)   std:      1046.76(2.91%)
> max:      24035.00        max:      37266.00
> min:      22108.00        min:      34149.00
> 
> In summary, MADV_FREE is about 2 time faster than MADV_DONTNEED.
> 
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> Cc: Linux API <linux-api@vger.kernel.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Jason Evans <je@fb.com>
> Cc: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
> Acked-by: Rik van Riel <riel@redhat.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  include/linux/rmap.h                   |   9 ++-
>  include/linux/vm_event_item.h          |   1 +
>  include/uapi/asm-generic/mman-common.h |   1 +
>  mm/madvise.c                           | 135 +++++++++++++++++++++++++++++++++
>  mm/rmap.c                              |  42 +++++++++-
>  mm/vmscan.c                            |  40 ++++++++--
>  mm/vmstat.c                            |   1 +
>  7 files changed, 217 insertions(+), 12 deletions(-)
> 

...

> @@ -251,6 +260,124 @@ static long madvise_willneed(struct vm_area_struct *vma,
>  	return 0;
>  }
>  
> +static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> +				unsigned long end, struct mm_walk *walk)
> +
> +{
> +	struct madvise_free_private *fp = walk->private;
> +	struct mmu_gather *tlb = fp->tlb;
> +	struct mm_struct *mm = tlb->mm;
> +	struct vm_area_struct *vma = fp->vma;
> +	spinlock_t *ptl;
> +	pte_t *pte, ptent;
> +	struct page *page;
> +
> +	split_huge_page_pmd(vma, addr, pmd);
> +	if (pmd_trans_unstable(pmd))
> +		return 0;
> +
> +	pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
> +	arch_enter_lazy_mmu_mode();
> +	for (; addr != end; pte++, addr += PAGE_SIZE) {
> +		ptent = *pte;
> +
> +		if (pte_none(ptent))
> +			continue;

The check is redundant: all pte_none() entries are also !pte_present().

> +
> +		if (!pte_present(ptent))
> +			continue;
> +
> +		page = vm_normal_page(vma, addr, ptent);
> +		if (page && PageSwapCache(page)) {
> +			if (trylock_page(page)) {
> +				if (try_to_free_swap(page))
> +					ClearPageDirty(page);
> +				unlock_page(page);
> +			} else
> +				continue;
> +		}

Is it safe to touch non-vm_normal entries? I would suggest to put
  if (!page)
	  continue;
instead.

> +		/*
> +		 * Some of architecture(ex, PPC) don't update TLB
> +		 * with set_pte_at and tlb_remove_tlb_entry so for
> +		 * the portability, remap the pte with old|clean
> +		 * after pte clearing.
> +		 */
> +		ptent = ptep_get_and_clear_full(mm, addr, pte,
> +						tlb->fullmm);
> +		ptent = pte_mkold(ptent);
> +		ptent = pte_mkclean(ptent);
> +		set_pte_at(mm, addr, pte, ptent);
> +		tlb_remove_tlb_entry(tlb, pte, addr);
> +	}
> +	arch_leave_lazy_mmu_mode();
> +	pte_unmap_unlock(pte - 1, ptl);
> +	cond_resched();
> +	return 0;
> +}
> +
> +static void madvise_free_page_range(struct mmu_gather *tlb,
> +			     struct vm_area_struct *vma,
> +			     unsigned long addr, unsigned long end)
> +{
> +	struct madvise_free_private fp = {
> +		.vma = vma,
> +		.tlb = tlb,
> +	};
> +
> +	struct mm_walk free_walk = {
> +		.pmd_entry = madvise_free_pte_range,
> +		.mm = vma->vm_mm,
> +		.private = &fp,
> +	};
> +
> +	BUG_ON(addr >= end);
> +	tlb_start_vma(tlb, vma);
> +	walk_page_range(addr, end, &free_walk);
> +	tlb_end_vma(tlb, vma);
> +}
> +
> +static int madvise_free_single_vma(struct vm_area_struct *vma,
> +			unsigned long start_addr, unsigned long end_addr)
> +{
> +	unsigned long start, end;
> +	struct mm_struct *mm = vma->vm_mm;
> +	struct mmu_gather tlb;
> +
> +	if (vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP))
> +		return -EINVAL;

VM_MIXEDMAP? VM_IO? Should it be whitelist instead?

> +
> +	/* MADV_FREE works for only anon vma at the moment */
> +	if (vma->vm_file)
> +		return -EINVAL;
> +
> +	start = max(vma->vm_start, start_addr);
> +	if (start >= vma->vm_end)
> +		return -EINVAL;
> +	end = min(vma->vm_end, end_addr);
> +	if (end <= vma->vm_start)
> +		return -EINVAL;
> +
> +	lru_add_drain();
> +	tlb_gather_mmu(&tlb, mm, start, end);
> +	update_hiwater_rss(mm);
> +
> +	mmu_notifier_invalidate_range_start(mm, start, end);
> +	madvise_free_page_range(&tlb, vma, start, end);
> +	mmu_notifier_invalidate_range_end(mm, start, end);
> +	tlb_finish_mmu(&tlb, start, end);
> +
> +	return 0;
> +}
> +
> +static long madvise_free(struct vm_area_struct *vma,
> +			     struct vm_area_struct **prev,
> +			     unsigned long start, unsigned long end)
> +{
> +	*prev = vma;
> +	return madvise_free_single_vma(vma, start, end);
> +}
> +
>  /*
>   * Application no longer needs these pages.  If the pages are dirty,
>   * it's OK to just throw them away.  The app will be more careful about
> @@ -381,6 +508,13 @@ 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_FREE:
> +		/*
> +		 * XXX: In this implementation, MADV_FREE works like
> +		 * MADV_DONTNEED on swapless system or full swap.
> +		 */
> +		if (get_nr_swap_pages() > 0)
> +			return madvise_free(vma, prev, start, end);

/* passthough */

>  	case MADV_DONTNEED:
>  		return madvise_dontneed(vma, prev, start, end);
>  	default:

...

> @@ -1186,6 +1210,19 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  		swp_entry_t entry = { .val = page_private(page) };
>  		pte_t swp_pte;
>  
> +		if (flags & TTU_FREE) {
> +			VM_BUG_ON_PAGE(PageSwapCache(page), page);
> +			if (dirty || PageDirty(page)) {
> +				set_pte_at(mm, address, pte, pteval);
> +				ret = SWAP_FAIL;
> +				goto out_unmap;

Hm. Again: do we really want stop here if caller asks for
TTU_FREE|TTU_UNMAP or should proceed?

> +			} else {
> +				/* It's a freeable page by MADV_FREE */
> +				dec_mm_counter(mm, MM_ANONPAGES);
> +				goto discard;
> +			}
> +		}
> +
>  		if (PageSwapCache(page)) {
>  			/*
>  			 * Store the swap location in the pte.
-- 
 Kirill A. Shutemov

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