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

Peter Xu
2020-09-21T21:17:55Z

(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