Re: [PATCH v2 3/3] mm: gup: pack has_pinned in MMF_HAS_PINNED

On Fri, 7 May 2021 at 20:36, Peter Xu peterx@redhat.com wrote:

From: Andrea Arcangeli aarcange@redhat.com

has_pinned 32bit can be packed in the MMF_HAS_PINNED bit as a noop cleanup.

Any atomic_inc/dec to the mm cacheline shared by all threads in pin-fast would reintroduce a loss of SMP scalability to pin-fast, so there's no future potential usefulness to keep an atomic in the mm for this.

set_bit(MMF_HAS_PINNED) will be theoretically a bit slower than WRITE_ONCE (atomic_set is equivalent to WRITE_ONCE), but the set_bit (just like atomic_set after this commit) has to be still issued only once per "mm", so the difference between the two will be lost in the noise.

will-it-scale "mmap2" shows no change in performance with enterprise config as expected.

will-it-scale "pin_fast" retains the > 4000% SMP scalability performance improvement against upstream as expected.

This is a noop as far as overall performance and SMP scalability are concerned.

Signed-off-by: Andrea Arcangeli aarcange@redhat.com [peterx: Fix build for task_mmu.c, introduce mm_set_has_pinned_flag, fix comment here and there] Signed-off-by: Peter Xu peterx@redhat.com

---

fs/proc/task_mmu.c | 2 +- include/linux/mm.h | 2 +- include/linux/mm_types.h | 10 ---------- include/linux/sched/coredump.h | 8 ++++++++ kernel/fork.c | 1 - mm/gup.c | 19 +++++++++++++++---- 6 files changed, 25 insertions(+), 17 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 4c95cc57a66a8..6144571942db9 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1049,7 +1049,7 @@ static inline bool pte_is_pinned(struct vm_area_struct *vma, unsigned long addr, return false; if (!is_cow_mapping(vma->vm_flags)) return false;

•   if (likely(!atomic_read(&vma->vm_mm->has_pinned)))
  

•   if (likely(!test_bit(MMF_HAS_PINNED, &vma->vm_mm->flags)))
            return false;
    page = vm_normal_page(vma, addr, pte);
    if (!page)
  

diff --git a/include/linux/mm.h b/include/linux/mm.h index d6790ab0cf575..94dc84f6d8658 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1331,7 +1331,7 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma, if (!is_cow_mapping(vma->vm_flags)) return false;

•   if (!atomic_read(&vma->vm_mm->has_pinned))
  

•   if (!test_bit(MMF_HAS_PINNED, &vma->vm_mm->flags))
            return false;
  
    return page_maybe_dma_pinned(page);
  

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 6613b26a88946..15d79858fadbd 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -435,16 +435,6 @@ struct mm_struct { */ atomic_t mm_count;

•           /**
  

•            * @has_pinned: Whether this mm has pinned any pages.  This can
  

•            * be either replaced in the future by @pinned_vm when it
  

•            * becomes stable, or grow into a counter on its own. We're
  

•            * aggresive on this bit now - even if the pinned pages were
  

•            * unpinned later on, we'll still keep this bit set for the
  

•            * lifecycle of this mm just for simplicity.
  

•            */
  

•           atomic_t has_pinned;
  

•           /**
             * @write_protect_seq: Locked when any thread is write
             * protecting pages mapped by this mm to enforce a later COW,
  

diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h index dfd82eab29025..4d9e3a6568758 100644 --- a/include/linux/sched/coredump.h +++ b/include/linux/sched/coredump.h @@ -73,6 +73,14 @@ static inline int get_dumpable(struct mm_struct *mm) #define MMF_OOM_VICTIM 25 /* mm is the oom victim */ #define MMF_OOM_REAP_QUEUED 26 /* mm was queued for oom_reaper */ #define MMF_MULTIPROCESS 27 /* mm is shared between processes */ +/*

• • MMF_HAS_PINNED: Whether this mm has pinned any pages. This can be either
• • replaced in the future by mm.pinned_vm when it becomes stable, or grow into
• • a counter on its own. We're aggresive on this bit for now: even if the
• • pinned pages were unpinned later on, we'll still keep this bit set for the
• • lifecycle of this mm, just for simplicity.
• */

+#define MMF_HAS_PINNED 28 /* FOLL_PIN has run, never cleared */ #define MMF_DISABLE_THP_MASK (1 << MMF_DISABLE_THP)

#define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\ diff --git a/kernel/fork.c b/kernel/fork.c index 502dc046fbc62..a71e73707ef59 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1026,7 +1026,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p, mm_pgtables_bytes_init(mm); mm->map_count = 0; mm->locked_vm = 0;

•   atomic_set(&mm->has_pinned, 0);
    atomic64_set(&mm->pinned_vm, 0);
    memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
    spin_lock_init(&mm->page_table_lock);
  

diff --git a/mm/gup.c b/mm/gup.c index 9933bc5c2eff2..bb130723a6717 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1270,6 +1270,17 @@ int fixup_user_fault(struct mm_struct *mm, } EXPORT_SYMBOL_GPL(fixup_user_fault);

+/*

• • Set the MMF_HAS_PINNED if not set yet; after set it'll be there for the mm's
• • lifecycle. Avoid setting the bit unless necessary, or it might cause write
• • cache bouncing on large SMP machines for concurrent pinned gups.
• */

+static inline void mm_set_has_pinned_flag(unsigned long *mm_flags) +{

•   if (!test_bit(MMF_HAS_PINNED, mm_flags))
  

•           set_bit(MMF_HAS_PINNED, mm_flags);
  

+}

/*

• Please note that this function, unlike __get_user_pages will not
• return 0 for nr_pages > 0 without FOLL_NOWAIT

@@ -1292,8 +1303,8 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm, BUG_ON(*locked != 1); }

•   if ((flags & FOLL_PIN) && !atomic_read(&mm->has_pinned))
  

•           atomic_set(&mm->has_pinned, 1);
  

•   if (flags & FOLL_PIN)
  

•           mm_set_has_pinned_flag(&mm->flags);
  
    /*
     * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
  

@@ -2617,8 +2628,8 @@ static int internal_get_user_pages_fast(unsigned long start, FOLL_FAST_ONLY))) return -EINVAL;

•   if ((gup_flags & FOLL_PIN) && !atomic_read(&current->mm->has_pinned))
  

•           atomic_set(&current->mm->has_pinned, 1);
  

•   if (gup_flags & FOLL_PIN)
  

•           mm_set_has_pinned_flag(&current->mm->flags);
  

Linux next tag next-20210512 builds failed on arm, riscv, mips and sh for the tinyconfig and allnoconfig due this patch.

arm, mips, riscv and sh (tinyconfig) with gcc-8 arm, mips, riscv and sh (allnoconfig) with gcc-8 arm, mips, riscv and sh (tinyconfig) with gcc-9 arm, mips, riscv and sh (allnoconfig) with gcc-9 arm, mips, riscv and sh (tinyconfig) with gcc-10 arm, mips, riscv and sh (allnoconfig) with gcc-10

mm/gup.c: In function 'internal_get_user_pages_fast': mm/gup.c:2698:3: error: implicit declaration of function 'mm_set_has_pinned_flag' [-Werror=implicit-function-declaration] 2698 | mm_set_has_pinned_flag(&current->mm->flags); | ^~~~~~~~~~~~~~~~~~~~~~ cc1: some warnings being treated as errors make[2]: *** [/builds/linux/scripts/Makefile.build:273: mm/gup.o] Error 1

Reported-by: Naresh Kamboju naresh.kamboju@linaro.org

#regzb introduced: 354a2e3604e2 ("mm: gup: pack has_pinned in MMF_HAS_PINNED")

Build url: https://gitlab.com/Linaro/lkft/mirrors/next/linux-next/-/jobs/1255567072#L31...

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