On Tue, Jul 14, 2020 at 10:40:57AM +0200, Peter Zijlstra wrote:

On Tue, Jul 14, 2020 at 12:02:16AM -0700, ira.weiny@intel.com wrote:

quoted

+static pgprot_t dev_protection_enable_get(struct dev_pagemap *pgmap, pgprot_t prot)
+{
+	if (pgmap->flags & PGMAP_PROT_ENABLED && dev_page_pkey != PKEY_INVALID) {
+		pgprotval_t val = pgprot_val(prot);
+
+		mutex_lock(&dev_prot_enable_lock);
+		dev_protection_enable++;
+		/* Only enable the static branch 1 time */
+		if (dev_protection_enable == 1)
+			static_branch_enable(&dev_protection_static_key);
+		mutex_unlock(&dev_prot_enable_lock);
+
+		prot = __pgprot(val | _PAGE_PKEY(dev_page_pkey));
+	}
+	return prot;
+}
+
+static void dev_protection_enable_put(struct dev_pagemap *pgmap)
+{
+	if (pgmap->flags & PGMAP_PROT_ENABLED && dev_page_pkey != PKEY_INVALID) {
+		mutex_lock(&dev_prot_enable_lock);
+		dev_protection_enable--;
+		if (dev_protection_enable == 0)
+			static_branch_disable(&dev_protection_static_key);
+		mutex_unlock(&dev_prot_enable_lock);
+	}
+}

That's an anti-pattern vs static_keys, I'm thinking you actually want
static_key_slow_{inc,dec}() instead of {enable,disable}().

Thanks.  I'll go read the doc for those as I'm not familiar with them.

Ira