[PATCH v6 02/17] zram: permit preemption with active compression stream

[Sergey Senozhatsky]

Currently, per-CPU stream access is done from a non-preemptible
(atomic) section, which imposes the same atomicity requirements on
compression backends as entry spin-lock, and makes it impossible
to use algorithms that can schedule/wait/sleep during compression
and decompression.

Switch to preemptible per-CPU model, similar to the one used
in zswap.  Instead of a per-CPU local lock, each stream carries
a mutex which is locked throughout entire time zram uses it
for compression or decompression, so that cpu-dead event waits
for zram to stop using a particular per-CPU stream and release
it.

Suggested-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
---
 drivers/block/zram/zcomp.c    | 41 +++++++++++++++++++++++++----------
 drivers/block/zram/zcomp.h    |  6 ++---
 drivers/block/zram/zram_drv.c | 20 ++++++++---------
 3 files changed, 42 insertions(+), 25 deletions(-)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index bb514403e305..53e4c37441be 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -6,7 +6,7 @@
 #include <linux/slab.h>
 #include <linux/wait.h>
 #include <linux/sched.h>
-#include <linux/cpu.h>
+#include <linux/cpuhotplug.h>
 #include <linux/crypto.h>
 #include <linux/vmalloc.h>
 
@@ -109,13 +109,29 @@ ssize_t zcomp_available_show(const char *comp, char *buf)
 
 struct zcomp_strm *zcomp_stream_get(struct zcomp *comp)
 {
-	local_lock(&comp->stream->lock);
-	return this_cpu_ptr(comp->stream);
+	for (;;) {
+		struct zcomp_strm *zstrm = raw_cpu_ptr(comp->stream);
+
+		/*
+		 * Inspired by zswap
+		 *
+		 * stream is returned with ->mutex locked which prevents
+		 * cpu_dead() from releasing this stream under us, however
+		 * there is still a race window between raw_cpu_ptr() and
+		 * mutex_lock(), during which we could have been migrated
+		 * from a CPU that has already destroyed its stream.  If
+		 * so then unlock and re-try on the current CPU.
+		 */
+		mutex_lock(&zstrm->lock);
+		if (likely(zstrm->buffer))
+			return zstrm;
+		mutex_unlock(&zstrm->lock);
+	}
 }
 
-void zcomp_stream_put(struct zcomp *comp)
+void zcomp_stream_put(struct zcomp_strm *zstrm)
 {
-	local_unlock(&comp->stream->lock);
+	mutex_unlock(&zstrm->lock);
 }
 
 int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
@@ -151,12 +167,9 @@ int zcomp_decompress(struct zcomp *comp, struct zcomp_strm *zstrm,
 int zcomp_cpu_up_prepare(unsigned int cpu, struct hlist_node *node)
 {
 	struct zcomp *comp = hlist_entry(node, struct zcomp, node);
-	struct zcomp_strm *zstrm;
+	struct zcomp_strm *zstrm = per_cpu_ptr(comp->stream, cpu);
 	int ret;
 
-	zstrm = per_cpu_ptr(comp->stream, cpu);
-	local_lock_init(&zstrm->lock);
-
 	ret = zcomp_strm_init(comp, zstrm);
 	if (ret)
 		pr_err("Can't allocate a compression stream\n");
@@ -166,16 +179,17 @@ int zcomp_cpu_up_prepare(unsigned int cpu, struct hlist_node *node)
 int zcomp_cpu_dead(unsigned int cpu, struct hlist_node *node)
 {
 	struct zcomp *comp = hlist_entry(node, struct zcomp, node);
-	struct zcomp_strm *zstrm;
+	struct zcomp_strm *zstrm = per_cpu_ptr(comp->stream, cpu);
 
-	zstrm = per_cpu_ptr(comp->stream, cpu);
+	mutex_lock(&zstrm->lock);
 	zcomp_strm_free(comp, zstrm);
+	mutex_unlock(&zstrm->lock);
 	return 0;
 }
 
 static int zcomp_init(struct zcomp *comp, struct zcomp_params *params)
 {
-	int ret;
+	int ret, cpu;
 
 	comp->stream = alloc_percpu(struct zcomp_strm);
 	if (!comp->stream)
@@ -186,6 +200,9 @@ static int zcomp_init(struct zcomp *comp, struct zcomp_params *params)
 	if (ret)
 		goto cleanup;
 
+	for_each_possible_cpu(cpu)
+		mutex_init(&per_cpu_ptr(comp->stream, cpu)->lock);
+
 	ret = cpuhp_state_add_instance(CPUHP_ZCOMP_PREPARE, &comp->node);
 	if (ret < 0)
 		goto cleanup;
diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
index ad5762813842..23b8236b9090 100644
--- a/drivers/block/zram/zcomp.h
+++ b/drivers/block/zram/zcomp.h
@@ -3,7 +3,7 @@
 #ifndef _ZCOMP_H_
 #define _ZCOMP_H_
 
-#include <linux/local_lock.h>
+#include <linux/mutex.h>
 
 #define ZCOMP_PARAM_NO_LEVEL	INT_MIN
 
@@ -31,7 +31,7 @@ struct zcomp_ctx {
 };
 
 struct zcomp_strm {
-	local_lock_t lock;
+	struct mutex lock;
 	/* compression buffer */
 	void *buffer;
 	struct zcomp_ctx ctx;
@@ -77,7 +77,7 @@ struct zcomp *zcomp_create(const char *alg, struct zcomp_params *params);
 void zcomp_destroy(struct zcomp *comp);
 
 struct zcomp_strm *zcomp_stream_get(struct zcomp *comp);
-void zcomp_stream_put(struct zcomp *comp);
+void zcomp_stream_put(struct zcomp_strm *zstrm);
 
 int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
 		   const void *src, unsigned int *dst_len);
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 65e16117f2db..ca439f3b1b9a 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1650,7 +1650,7 @@ static int read_compressed_page(struct zram *zram, struct page *page, u32 index)
 	ret = zcomp_decompress(zram->comps[prio], zstrm, src, size, dst);
 	kunmap_local(dst);
 	zs_unmap_object(zram->mem_pool, handle);
-	zcomp_stream_put(zram->comps[prio]);
+	zcomp_stream_put(zstrm);
 
 	return ret;
 }
@@ -1811,14 +1811,14 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
 	kunmap_local(mem);
 
 	if (unlikely(ret)) {
-		zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+		zcomp_stream_put(zstrm);
 		pr_err("Compression failed! err=%d\n", ret);
 		zs_free(zram->mem_pool, handle);
 		return ret;
 	}
 
 	if (comp_len >= huge_class_size) {
-		zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+		zcomp_stream_put(zstrm);
 		return write_incompressible_page(zram, page, index);
 	}
 
@@ -1842,7 +1842,7 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
 				   __GFP_HIGHMEM |
 				   __GFP_MOVABLE);
 	if (IS_ERR_VALUE(handle)) {
-		zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+		zcomp_stream_put(zstrm);
 		atomic64_inc(&zram->stats.writestall);
 		handle = zs_malloc(zram->mem_pool, comp_len,
 				   GFP_NOIO | __GFP_HIGHMEM |
@@ -1854,7 +1854,7 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
 	}
 
 	if (!zram_can_store_page(zram)) {
-		zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+		zcomp_stream_put(zstrm);
 		zs_free(zram->mem_pool, handle);
 		return -ENOMEM;
 	}
@@ -1862,7 +1862,7 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
 	dst = zs_map_object(zram->mem_pool, handle, ZS_MM_WO);
 
 	memcpy(dst, zstrm->buffer, comp_len);
-	zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+	zcomp_stream_put(zstrm);
 	zs_unmap_object(zram->mem_pool, handle);
 
 	zram_slot_lock(zram, index);
@@ -2021,7 +2021,7 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 		kunmap_local(src);
 
 		if (ret) {
-			zcomp_stream_put(zram->comps[prio]);
+			zcomp_stream_put(zstrm);
 			return ret;
 		}
 
@@ -2031,7 +2031,7 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 		/* Continue until we make progress */
 		if (class_index_new >= class_index_old ||
 		    (threshold && comp_len_new >= threshold)) {
-			zcomp_stream_put(zram->comps[prio]);
+			zcomp_stream_put(zstrm);
 			continue;
 		}
 
@@ -2089,13 +2089,13 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 			       __GFP_HIGHMEM |
 			       __GFP_MOVABLE);
 	if (IS_ERR_VALUE(handle_new)) {
-		zcomp_stream_put(zram->comps[prio]);
+		zcomp_stream_put(zstrm);
 		return PTR_ERR((void *)handle_new);
 	}
 
 	dst = zs_map_object(zram->mem_pool, handle_new, ZS_MM_WO);
 	memcpy(dst, zstrm->buffer, comp_len_new);
-	zcomp_stream_put(zram->comps[prio]);
+	zcomp_stream_put(zstrm);
 
 	zs_unmap_object(zram->mem_pool, handle_new);
 
-- 
2.48.1.601.g30ceb7b040-goog