[PATCH v2 1/4] mm: Introduce mm_struct.has_pinned

[Peter Xu]

(Commit message majorly 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().  This allows
cases that might drive up the page ref_count to avoid any penalty from handling
dma_pinned pages.

Future work is planned, to provide a more sophisticated solution, likely to
turn it into a real counter.  For now, make it atomic_t but use it as a boolean
for simplicity.

Suggested-by: Jason Gunthorpe <j...@ziepe.ca>
Signed-off-by: Peter Xu <pet...@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..ed028af3cb19 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -436,6 +436,16 @@ 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;
+
 #ifdef CONFIG_MMU
                atomic_long_t pgtables_bytes;   /* PTE page table pages */
 #endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 49677d668de4..e65d8192d080 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;
+       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 e5739a1974d5..238667445337 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)
+               atomic_set(&current->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)
+               atomic_set(&current->mm->has_pinned, 1);
+
        if (!(gup_flags & FOLL_FAST_ONLY))
                might_lock_read(&current->mm->mmap_lock);
 
-- 
2.26.2