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.

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

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Andrea Arcangeli <[e-mail]>
Signed-off-by: Peter Xu <[e-mail]>
Reviewed-by: John Hubbard <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: Kirill Shutemov <[e-mail]>
Cc: Kirill Tkhai <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/task_mmu.c

···
 		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)

include/linux/mm.h

···
 	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);

include/linux/mm_types.h

···
 		 */
 		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;
-
 #ifdef CONFIG_MMU
 		atomic_long_t pgtables_bytes;	/* PTE page table pages */
 #endif

include/linux/sched/coredump.h

···
 #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 |\

kernel/fork.c

···
 	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);

mm/gup.c

···
 }
 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)
···
 		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
···
 				       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);