[PATCH 1/5] mm: Introduce mm_struct.has_pinned

[Peter Xu <peterx@redhat.com>]

(Commit message collected from Jason Gunthorpe)

Reduce the chance of false positive from page_maybe_dma_pinned() by keeping
track if the mm_struct has ever been used with pin_user_pages(). mm_structs
that have never been passed to pin_user_pages() cannot have a positive
page_maybe_dma_pinned() by definition. This allows cases that might drive up
the page ref_count to avoid any penalty from handling dma_pinned pages.

Due to complexities with unpining this trivial version is a permanent sticky
bit, future work will be needed to make this a counter.

Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/mm_types.h | 10 ++++++++++
 kernel/fork.c            |  1 +
 mm/gup.c                 |  6 ++++++
 3 files changed, 17 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 496c3ff97cce..6f291f8b74c6 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -441,6 +441,16 @@ struct mm_struct {
 #endif
 		int map_count;			/* number of VMAs */
 
+		/**
+		 * @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.
+		 */
+		int has_pinned;
+
 		spinlock_t page_table_lock; /* Protects page tables and some
 					     * counters
 					     */
diff --git a/kernel/fork.c b/kernel/fork.c
index 49677d668de4..7237d418e7b5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1011,6 +1011,7 @@ 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;
+	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 e5739a1974d5..2d9019bf1773 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1255,6 +1255,9 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
 		BUG_ON(*locked != 1);
 	}
 
+	if (flags & FOLL_PIN)
+		WRITE_ONCE(mm->has_pinned, 1);
+
 	/*
 	 * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
 	 * is to set FOLL_GET if the caller wants pages[] filled in (but has
@@ -2660,6 +2663,9 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 				       FOLL_FAST_ONLY)))
 		return -EINVAL;
 
+	if (gup_flags & FOLL_PIN)
+		WRITE_ONCE(current->mm->has_pinned, 1);
+
 	if (!(gup_flags & FOLL_FAST_ONLY))
 		might_lock_read(&current->mm->mmap_lock);
 
-- 
2.26.2