[PATCH v3] mm: Enable suspend-only swap spaces

[Evan Green <evgreen@chromium.org>]

Currently it's not possible to enable hibernation without also enabling
generic swap for a given swap area. These two use cases are not the
same. For example there may be users who want to enable hibernation,
but whose drives don't have the write endurance for generic swap
activities. Swap and hibernate also have different security/integrity
requirements, prompting folks to possibly set up something like block-level
integrity for swap and image-level integrity for hibernate. Keeping swap
and hibernate separate in these cases becomes not just a matter of
preference, but correctness.

Add a new SWAP_FLAG_NOSWAP that adds a swap region but refuses to allow
generic swapping to it. This region can still be wired up for use in
suspend-to-disk activities, but will never have regular pages swapped to
it. This flag will be passed in by utilities like swapon(8), usage would
probably look something like: swapon -o noswap /dev/sda2.

Swap regions with SWAP_FLAG_NOSWAP set will not appear in /proc/meminfo
under SwapTotal and SwapFree, since they are not usable as general swap.

Signed-off-by: Evan Green <evgreen@chromium.org>
---

Changes in v3:
 - Updated commit message with additional explanation [Andrew]

Changes in v2:
 - NOSWAP regions should not contribute to Swap stats in /proc/meminfo.
   [David]
 - Adjusted comment of SWAP_FLAG_NOSWAP [Pavel]
 - Note: Opted not to take Pavel's tag since enough has changed in this
   revision to warrant another look.
 - Call swap_entry_free() in swap_free to avoid NOSWAP leaks back into
   the general pool via swap_slots_cache [me].

 include/linux/swap.h |  4 +++-
 mm/swapfile.c        | 52 +++++++++++++++++++++++++++++++-------------
 2 files changed, 40 insertions(+), 16 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index cdf0957a88a49a..5e1d80be84bb02 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -28,10 +28,11 @@ struct pagevec;
 #define SWAP_FLAG_DISCARD	0x10000 /* enable discard for swap */
 #define SWAP_FLAG_DISCARD_ONCE	0x20000 /* discard swap area at swapon-time */
 #define SWAP_FLAG_DISCARD_PAGES 0x40000 /* discard page-clusters after use */
+#define SWAP_FLAG_NOSWAP	0x80000 /* use only for hibernate, not swap */
 
 #define SWAP_FLAGS_VALID	(SWAP_FLAG_PRIO_MASK | SWAP_FLAG_PREFER | \
 				 SWAP_FLAG_DISCARD | SWAP_FLAG_DISCARD_ONCE | \
-				 SWAP_FLAG_DISCARD_PAGES)
+				 SWAP_FLAG_DISCARD_PAGES | SWAP_FLAG_NOSWAP)
 #define SWAP_BATCH 64
 
 static inline int current_is_kswapd(void)
@@ -182,6 +183,7 @@ enum {
 	SWP_PAGE_DISCARD = (1 << 10),	/* freed swap page-cluster discards */
 	SWP_STABLE_WRITES = (1 << 11),	/* no overwrite PG_writeback pages */
 	SWP_SYNCHRONOUS_IO = (1 << 12),	/* synchronous IO is efficient */
+	SWP_NOSWAP	= (1 << 13),	/* use only for suspend, not swap */
 					/* add others here before... */
 	SWP_SCANNING	= (1 << 14),	/* refcount in scan_swap_map */
 };
diff --git a/mm/swapfile.c b/mm/swapfile.c
index e3dcaeecc50f54..ca0e5e5d1f3074 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -697,7 +697,8 @@ static void swap_range_alloc(struct swap_info_struct *si, unsigned long offset,
 	if (si->inuse_pages == si->pages) {
 		si->lowest_bit = si->max;
 		si->highest_bit = 0;
-		del_from_avail_list(si);
+		if (!(si->flags & SWP_NOSWAP))
+			del_from_avail_list(si);
 	}
 }
 
@@ -726,10 +727,12 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 		bool was_full = !si->highest_bit;
 
 		WRITE_ONCE(si->highest_bit, end);
-		if (was_full && (si->flags & SWP_WRITEOK))
+		if (was_full &&
+		    ((si->flags & (SWP_WRITEOK | SWP_NOSWAP)) == SWP_WRITEOK))
 			add_to_avail_list(si);
 	}
-	atomic_long_add(nr_entries, &nr_swap_pages);
+	if (!(si->flags & SWP_NOSWAP))
+		atomic_long_add(nr_entries, &nr_swap_pages);
 	si->inuse_pages -= nr_entries;
 	if (si->flags & SWP_BLKDEV)
 		swap_slot_free_notify =
@@ -1078,6 +1081,9 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
 			WARN(!(si->flags & SWP_WRITEOK),
 			     "swap_info %d in list but !SWP_WRITEOK\n",
 			     si->type);
+			WARN((si->flags & SWP_NOSWAP),
+			     "swap_info %d in list but SWP_NOSWAP\n",
+			     si->type);
 			__del_from_avail_list(si);
 			spin_unlock(&si->lock);
 			goto nextsi;
@@ -1338,8 +1344,12 @@ void swap_free(swp_entry_t entry)
 	struct swap_info_struct *p;
 
 	p = _swap_info_get(entry);
-	if (p)
-		__swap_entry_free(p, entry);
+	if (p) {
+		if (p->flags & SWP_NOSWAP)
+			swap_entry_free(p, entry);
+		else
+			__swap_entry_free(p, entry);
+	}
 }
 
 /*
@@ -1783,8 +1793,10 @@ swp_entry_t get_swap_page_of_type(int type)
 
 	/* This is called for allocating swap entry, not cache */
 	spin_lock(&si->lock);
-	if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry))
-		atomic_long_dec(&nr_swap_pages);
+	if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry)) {
+		if (!(si->flags & SWP_NOSWAP))
+			atomic_long_dec(&nr_swap_pages);
+	}
 	spin_unlock(&si->lock);
 fail:
 	return entry;
@@ -2454,8 +2466,6 @@ static void setup_swap_info(struct swap_info_struct *p, int prio,
 static void _enable_swap_info(struct swap_info_struct *p)
 {
 	p->flags |= SWP_WRITEOK;
-	atomic_long_add(p->pages, &nr_swap_pages);
-	total_swap_pages += p->pages;
 
 	assert_spin_locked(&swap_lock);
 	/*
@@ -2469,7 +2479,11 @@ static void _enable_swap_info(struct swap_info_struct *p)
 	 * swap_info_struct.
 	 */
 	plist_add(&p->list, &swap_active_head);
-	add_to_avail_list(p);
+	if (!(p->flags & SWP_NOSWAP)) {
+		atomic_long_add(p->pages, &nr_swap_pages);
+		total_swap_pages += p->pages;
+		add_to_avail_list(p);
+	}
 }
 
 static void enable_swap_info(struct swap_info_struct *p, int prio,
@@ -2564,7 +2578,9 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		spin_unlock(&swap_lock);
 		goto out_dput;
 	}
-	del_from_avail_list(p);
+	if (!(p->flags & SWP_NOSWAP))
+		del_from_avail_list(p);
+
 	spin_lock(&p->lock);
 	if (p->prio < 0) {
 		struct swap_info_struct *si = p;
@@ -2581,8 +2597,10 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		least_priority++;
 	}
 	plist_del(&p->list, &swap_active_head);
-	atomic_long_sub(p->pages, &nr_swap_pages);
-	total_swap_pages -= p->pages;
+	if (!(p->flags & SWP_NOSWAP)) {
+		atomic_long_sub(p->pages, &nr_swap_pages);
+		total_swap_pages -= p->pages;
+	}
 	p->flags &= ~SWP_WRITEOK;
 	spin_unlock(&p->lock);
 	spin_unlock(&swap_lock);
@@ -3335,16 +3353,20 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	if (swap_flags & SWAP_FLAG_PREFER)
 		prio =
 		  (swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT;
+
+	if (swap_flags & SWAP_FLAG_NOSWAP)
+		p->flags |= SWP_NOSWAP;
 	enable_swap_info(p, prio, swap_map, cluster_info, frontswap_map);
 
-	pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s%s\n",
+	pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s%s%s\n",
 		p->pages<<(PAGE_SHIFT-10), name->name, p->prio,
 		nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10),
 		(p->flags & SWP_SOLIDSTATE) ? "SS" : "",
 		(p->flags & SWP_DISCARDABLE) ? "D" : "",
 		(p->flags & SWP_AREA_DISCARD) ? "s" : "",
 		(p->flags & SWP_PAGE_DISCARD) ? "c" : "",
-		(frontswap_map) ? "FS" : "");
+		(frontswap_map) ? "FS" : "",
+		(p->flags & SWP_NOSWAP) ? "N" : "");
 
 	mutex_unlock(&swapon_mutex);
 	atomic_inc(&proc_poll_event);
-- 
2.31.0