Re: [PATCH 3/3] mm: gup: pack has_pinned in MMF_HAS_PINNED
From: John Hubbard
Date: Fri May 07 2021 - 02:43:05 EST
On 5/6/21 4:25 PM, Peter Xu wrote:
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
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.
It's nice that you spelled that out. I don't see any technical problems
with the diffs. There are a couple of tiny suggestions, below.
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
[peterx: Fix build for task_mmu.c]
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
fs/proc/task_mmu.c | 2 +-
include/linux/mm.h | 2 +-
include/linux/mm_types.h | 10 ----------
include/linux/sched/coredump.h | 1 +
kernel/fork.c | 1 -
mm/gup.c | 9 +++++----
6 files changed, 8 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..bf45badd63e6d 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -73,6 +73,7 @@ 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 */
+#define MMF_HAS_PINNED 28 /* FOLL_PIN has run, never cleared */
How about this instead, so that we effectively retain the comment block
that is otherwise being deleted from mm.h:
/*
* 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 ran. 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 8b513e1723b45..78416b0909873 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1292,8 +1292,8 @@ 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 && !test_bit(MMF_HAS_PINNED, &mm->flags))
+ set_bit(MMF_HAS_PINNED, &mm->flags);
I expect this suggestion to be controversial, but I'm going to float it
anyway. The above is a little less clear than it used to be, *and* it is
in two places so far, so how about factoring out a tiny subroutine, like this:
diff --git a/mm/gup.c b/mm/gup.c
index 036ab0de9457..2dc001a7c850 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1270,6 +1270,16 @@ int fixup_user_fault(struct mm_struct *mm,
}
EXPORT_SYMBOL_GPL(fixup_user_fault);
+static void set_mm_has_pinned_flag(unsigned long *mm_flags)
+{
+ /*
+ * Avoid setting the bit unless necessary. This matters a lot with
+ * large SMP machines.
+ */
+ 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 +1302,8 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
BUG_ON(*locked != 1);
}
- if (flags & FOLL_PIN && !test_bit(MMF_HAS_PINNED, &mm->flags))
- set_bit(MMF_HAS_PINNED, &mm->flags);
+ if (flags & FOLL_PIN)
+ set_mm_has_pinned_flag(&mm->flags);
...which is now very readable, once again.
/*
* FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
@@ -2617,8 +2617,9 @@ static int internal_get_user_pages_fast(unsigned long start,
FOLL_FAST_ONLY)))
return -EINVAL;
- if (gup_flags & FOLL_PIN && !atomic_read(¤t->mm->has_pinned))
- atomic_set(¤t->mm->has_pinned, 1);
+ if (gup_flags & FOLL_PIN &&
+ !test_bit(MMF_HAS_PINNED, ¤t->mm->flags))
+ set_bit(MMF_HAS_PINNED, ¤t->mm->flags);
if (!(gup_flags & FOLL_FAST_ONLY))
might_lock_read(¤t->mm->mmap_lock);
thanks,
--
John Hubbard
NVIDIA