[PATCH 8/8] mm/zswap: Use local lock to protect per-CPU data

[Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>]

From: "Luis Claudio R. Goncalves" <lgoncalv@xxxxxxxxxx>

zwap uses per-CPU compression. The per-CPU data pointer is acquired with
get_cpu_ptr() which implicitly disables preemption. It allocates
memory inside the preempt disabled region which conflicts with the
PREEMPT_RT semantics.

Replace the implicit preemption control with an explicit local lock.
This allows RT kernels to substitute it with a real per CPU lock, which
serializes the access but keeps the code section preemptible. On non RT
kernels this maps to preempt_disable() as before, i.e. no functional
change.

[bigeasy: Use local_lock(), additional hunks, patch description]

Cc: Seth Jennings <sjenning@xxxxxxxxxx>
Cc: Dan Streetman <ddstreet@xxxxxxxx>
Cc: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 mm/zswap.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index fbb782924ccc5..1db2ad941e501 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -18,6 +18,7 @@
 #include <linux/highmem.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/locallock.h>
 #include <linux/types.h>
 #include <linux/atomic.h>
 #include <linux/frontswap.h>
@@ -388,6 +389,8 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
 * per-cpu code
 **********************************/
 static DEFINE_PER_CPU(u8 *, zswap_dstmem);
+/* Used for zswap_dstmem and tfm */
+static DEFINE_LOCAL_LOCK(zswap_cpu_lock);
 
 static int zswap_dstmem_prepare(unsigned int cpu)
 {
@@ -919,10 +922,11 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
 		dlen = PAGE_SIZE;
 		src = (u8 *)zhdr + sizeof(struct zswap_header);
 		dst = kmap_atomic(page);
-		tfm = *get_cpu_ptr(entry->pool->tfm);
+		local_lock(zswap_cpu_lock);
+		tfm = *this_cpu_ptr(entry->pool->tfm);
 		ret = crypto_comp_decompress(tfm, src, entry->length,
 					     dst, &dlen);
-		put_cpu_ptr(entry->pool->tfm);
+		local_unlock(zswap_cpu_lock);
 		kunmap_atomic(dst);
 		BUG_ON(ret);
 		BUG_ON(dlen != PAGE_SIZE);
@@ -1074,12 +1078,12 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	}
 
 	/* compress */
-	dst = get_cpu_var(zswap_dstmem);
-	tfm = *get_cpu_ptr(entry->pool->tfm);
+	local_lock(zswap_cpu_lock);
+	dst = *this_cpu_ptr(&zswap_dstmem);
+	tfm = *this_cpu_ptr(entry->pool->tfm);
 	src = kmap_atomic(page);
 	ret = crypto_comp_compress(tfm, src, PAGE_SIZE, dst, &dlen);
 	kunmap_atomic(src);
-	put_cpu_ptr(entry->pool->tfm);
 	if (ret) {
 		ret = -EINVAL;
 		goto put_dstmem;
@@ -1103,7 +1107,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	memcpy(buf, &zhdr, hlen);
 	memcpy(buf + hlen, dst, dlen);
 	zpool_unmap_handle(entry->pool->zpool, handle);
-	put_cpu_var(zswap_dstmem);
+	local_unlock(zswap_cpu_lock);
 
 	/* populate entry */
 	entry->offset = offset;
@@ -1131,7 +1135,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	return 0;
 
 put_dstmem:
-	put_cpu_var(zswap_dstmem);
+	local_unlock(zswap_cpu_lock);
 	zswap_pool_put(entry->pool);
 freepage:
 	zswap_entry_cache_free(entry);
@@ -1176,9 +1180,10 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 	if (zpool_evictable(entry->pool->zpool))
 		src += sizeof(struct zswap_header);
 	dst = kmap_atomic(page);
-	tfm = *get_cpu_ptr(entry->pool->tfm);
+	local_lock(zswap_cpu_lock);
+	tfm = *this_cpu_ptr(entry->pool->tfm);
 	ret = crypto_comp_decompress(tfm, src, entry->length, dst, &dlen);
-	put_cpu_ptr(entry->pool->tfm);
+	local_unlock(zswap_cpu_lock);
 	kunmap_atomic(dst);
 	zpool_unmap_handle(entry->pool->zpool, entry->handle);
 	BUG_ON(ret);
-- 
2.26.2