mm: gup: pack has_pinned in MMF_HAS_PINNED

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.

[peterx@redhat.com: pack has_pinned in MMF_HAS_PINNED]
  Link: https://lkml.kernel.org/r/YJqWESqyxa8OZA+2@t490s
[akpm@linux-foundation.org: coding style fixes]
[peterx@redhat.com: fix build for task_mmu.c, introduce mm_set_has_pinned_flag, fix comments]

Link: https://lkml.kernel.org/r/20210507150553.208763-4-peterx@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Kirill Shutemov <kirill@shutemov.name>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 4 deletions

diff --git a/mm/gup.c b/mm/gup.c
index a6c20a7b3c49..8651309f8ec3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -420,6 +420,17 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
 }
 EXPORT_SYMBOL(unpin_user_pages);
 
+/*
+ * 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);
+}
+
 #ifdef CONFIG_MMU
 static struct page *no_page_table(struct vm_area_struct *vma,
 		unsigned int flags)
@@ -1320,8 +1331,8 @@ 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
@@ -2641,8 +2652,8 @@ 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);
 
 	if (!(gup_flags & FOLL_FAST_ONLY))
 		might_lock_read(&current->mm->mmap_lock);