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

From: Andrea Arcangeli <redacted>

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 <redacted>
[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);