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

Kirill A. Shutemov
2014-07-08T09:41:49Z

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