dm-pcache: add persistent cache target in device-mapper

This patch introduces dm-pcache, a new DM target that places a DAX-
capable persistent-memory device in front of any slower block device and
uses it as a high-throughput, low-latency  cache.

Design highlights
-----------------
- DAX data path – data is copied directly between DRAM and the pmem
  mapping, bypassing the block layer’s overhead.

- Segmented, crash-consistent layout
  - all layout metadata are dual-replicated CRC-protected.
  - atomic kset flushes; key replay on mount guarantees cache integrity
    even after power loss.

- Striped multi-tree index
  - Multi‑tree indexing for high parallelism.
  - overlap-resolution logic ensures non-intersecting cached extents.

- Background services
  - write-back worker flushes dirty keys in order, preserving backing-device
    crash consistency. This is important for checkpoint in cloud storage.
  - garbage collector reclaims clean segments when utilisation exceeds a
    tunable threshold.

- Data integrity – optional CRC32 on cached payload; metadata always protected.

Comparison with existing block-level caches
---------------------------------------------------------------------------------------------------------------------------------
| Feature                          | pcache (this patch)             | bcache                       | dm-writecache             |
|----------------------------------|---------------------------------|------------------------------|---------------------------|
| pmem access method               | DAX                             | bio (block I/O)              | DAX                       |
| Write latency (4 K rand-write)   | ~5 µs                           | ~20 µs                       | ~5 µs                     |
| Concurrency                      | multi subtree index             | global index tree            | single tree + wc_lock     |
| IOPS (4K randwrite, 32 numjobs)  | 2.1 M                           | 352 K                        | 283 K                     |
| Read-cache support               | YES                             | YES                          | NO                        |
| Deployment                       | no re-format of backend         | backend devices must be      | no re-format of backend   |
|                                  |                                 | reformatted                  |                           |
| Write-back ordering              | log-structured;                 | no ordering guarantee        | no ordering guarantee     |
|                                  | preserves app-IO-order          |                              |                           |
| Data integrity checks            | metadata + data CRC(optional)   | metadata CRC only            | none                      |
---------------------------------------------------------------------------------------------------------------------------------

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

1 files changed, 293 insertions, 0 deletions

diff --git a/drivers/md/dm-pcache/cache_segment.c b/drivers/md/dm-pcache/cache_segment.c
new file mode 100644
index 000000000000..8f9534ae04c3
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_segment.c
@@ -0,0 +1,293 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "cache_dev.h"
+#include "cache.h"
+#include "backing_dev.h"
+#include "dm_pcache.h"
+
+static inline struct pcache_segment_info *get_seg_info_addr(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_segment_info *seg_info_addr;
+	u32 seg_id = cache_seg->segment.seg_id;
+	void *seg_addr;
+
+	seg_addr = CACHE_DEV_SEGMENT(cache_seg->cache->cache_dev, seg_id);
+	seg_info_addr = seg_addr + PCACHE_SEG_INFO_SIZE * cache_seg->info_index;
+
+	return seg_info_addr;
+}
+
+static void cache_seg_info_write(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_segment_info *seg_info_addr;
+	struct pcache_segment_info *seg_info = &cache_seg->cache_seg_info;
+
+	mutex_lock(&cache_seg->info_lock);
+	seg_info->header.seq++;
+	seg_info->header.crc = pcache_meta_crc(&seg_info->header, sizeof(struct pcache_segment_info));
+
+	seg_info_addr = get_seg_info_addr(cache_seg);
+	memcpy_flushcache(seg_info_addr, seg_info, sizeof(struct pcache_segment_info));
+	pmem_wmb();
+
+	cache_seg->info_index = (cache_seg->info_index + 1) % PCACHE_META_INDEX_MAX;
+	mutex_unlock(&cache_seg->info_lock);
+}
+
+static int cache_seg_info_load(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_segment_info *cache_seg_info_addr_base, *cache_seg_info_addr;
+	struct pcache_cache_dev *cache_dev = cache_seg->cache->cache_dev;
+	struct dm_pcache *pcache = CACHE_DEV_TO_PCACHE(cache_dev);
+	u32 seg_id = cache_seg->segment.seg_id;
+	int ret = 0;
+
+	cache_seg_info_addr_base = CACHE_DEV_SEGMENT(cache_dev, seg_id);
+
+	mutex_lock(&cache_seg->info_lock);
+	cache_seg_info_addr = pcache_meta_find_latest(&cache_seg_info_addr_base->header,
+						sizeof(struct pcache_segment_info),
+						PCACHE_SEG_INFO_SIZE,
+						&cache_seg->cache_seg_info);
+	if (IS_ERR(cache_seg_info_addr)) {
+		ret = PTR_ERR(cache_seg_info_addr);
+		goto out;
+	} else if (!cache_seg_info_addr) {
+		ret = -EIO;
+		goto out;
+	}
+	cache_seg->info_index = cache_seg_info_addr - cache_seg_info_addr_base;
+out:
+	mutex_unlock(&cache_seg->info_lock);
+
+	if (ret)
+		pcache_dev_err(pcache, "can't read segment info of segment: %u, ret: %d\n",
+			      cache_seg->segment.seg_id, ret);
+	return ret;
+}
+
+static int cache_seg_ctrl_load(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_cache_seg_ctrl *cache_seg_ctrl = cache_seg->cache_seg_ctrl;
+	struct pcache_cache_seg_gen cache_seg_gen, *cache_seg_gen_addr;
+	int ret = 0;
+
+	mutex_lock(&cache_seg->ctrl_lock);
+	cache_seg_gen_addr = pcache_meta_find_latest(&cache_seg_ctrl->gen->header,
+					     sizeof(struct pcache_cache_seg_gen),
+					     sizeof(struct pcache_cache_seg_gen),
+					     &cache_seg_gen);
+	if (IS_ERR(cache_seg_gen_addr)) {
+		ret = PTR_ERR(cache_seg_gen_addr);
+		goto out;
+	}
+
+	if (!cache_seg_gen_addr) {
+		cache_seg->gen = 0;
+		cache_seg->gen_seq = 0;
+		cache_seg->gen_index = 0;
+		goto out;
+	}
+
+	cache_seg->gen = cache_seg_gen.gen;
+	cache_seg->gen_seq = cache_seg_gen.header.seq;
+	cache_seg->gen_index = (cache_seg_gen_addr - cache_seg_ctrl->gen);
+out:
+	mutex_unlock(&cache_seg->ctrl_lock);
+
+	return ret;
+}
+
+static inline struct pcache_cache_seg_gen *get_cache_seg_gen_addr(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_cache_seg_ctrl *cache_seg_ctrl = cache_seg->cache_seg_ctrl;
+
+	return (cache_seg_ctrl->gen + cache_seg->gen_index);
+}
+
+static void cache_seg_ctrl_write(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_cache_seg_gen cache_seg_gen;
+
+	mutex_lock(&cache_seg->ctrl_lock);
+	cache_seg_gen.gen = cache_seg->gen;
+	cache_seg_gen.header.seq = ++cache_seg->gen_seq;
+	cache_seg_gen.header.crc = pcache_meta_crc(&cache_seg_gen.header,
+						 sizeof(struct pcache_cache_seg_gen));
+
+	memcpy_flushcache(get_cache_seg_gen_addr(cache_seg), &cache_seg_gen, sizeof(struct pcache_cache_seg_gen));
+	pmem_wmb();
+
+	cache_seg->gen_index = (cache_seg->gen_index + 1) % PCACHE_META_INDEX_MAX;
+	mutex_unlock(&cache_seg->ctrl_lock);
+}
+
+static void cache_seg_ctrl_init(struct pcache_cache_segment *cache_seg)
+{
+	cache_seg->gen = 0;
+	cache_seg->gen_seq = 0;
+	cache_seg->gen_index = 0;
+	cache_seg_ctrl_write(cache_seg);
+}
+
+static int cache_seg_meta_load(struct pcache_cache_segment *cache_seg)
+{
+	int ret;
+
+	ret = cache_seg_info_load(cache_seg);
+	if (ret)
+		goto err;
+
+	ret = cache_seg_ctrl_load(cache_seg);
+	if (ret)
+		goto err;
+
+	return 0;
+err:
+	return ret;
+}
+
+/**
+ * cache_seg_set_next_seg - Sets the ID of the next segment
+ * @cache_seg: Pointer to the cache segment structure.
+ * @seg_id: The segment ID to set as the next segment.
+ *
+ * A pcache_cache allocates multiple cache segments, which are linked together
+ * through next_seg. When loading a pcache_cache, the first cache segment can
+ * be found using cache->seg_id, which allows access to all the cache segments.
+ */
+void cache_seg_set_next_seg(struct pcache_cache_segment *cache_seg, u32 seg_id)
+{
+	cache_seg->cache_seg_info.flags |= PCACHE_SEG_INFO_FLAGS_HAS_NEXT;
+	cache_seg->cache_seg_info.next_seg = seg_id;
+	cache_seg_info_write(cache_seg);
+}
+
+int cache_seg_init(struct pcache_cache *cache, u32 seg_id, u32 cache_seg_id,
+		   bool new_cache)
+{
+	struct pcache_cache_dev *cache_dev = cache->cache_dev;
+	struct pcache_cache_segment *cache_seg = &cache->segments[cache_seg_id];
+	struct pcache_segment_init_options seg_options = { 0 };
+	struct pcache_segment *segment = &cache_seg->segment;
+	int ret;
+
+	cache_seg->cache = cache;
+	cache_seg->cache_seg_id = cache_seg_id;
+	spin_lock_init(&cache_seg->gen_lock);
+	atomic_set(&cache_seg->refs, 0);
+	mutex_init(&cache_seg->info_lock);
+	mutex_init(&cache_seg->ctrl_lock);
+
+	/* init pcache_segment */
+	seg_options.type = PCACHE_SEGMENT_TYPE_CACHE_DATA;
+	seg_options.data_off = PCACHE_CACHE_SEG_CTRL_OFF + PCACHE_CACHE_SEG_CTRL_SIZE;
+	seg_options.seg_id = seg_id;
+	seg_options.seg_info = &cache_seg->cache_seg_info;
+	pcache_segment_init(cache_dev, segment, &seg_options);
+
+	cache_seg->cache_seg_ctrl = CACHE_DEV_SEGMENT(cache_dev, seg_id) + PCACHE_CACHE_SEG_CTRL_OFF;
+
+	if (new_cache) {
+		cache_dev_zero_range(cache_dev, CACHE_DEV_SEGMENT(cache_dev, seg_id),
+				     PCACHE_SEG_INFO_SIZE * PCACHE_META_INDEX_MAX +
+				     PCACHE_CACHE_SEG_CTRL_SIZE);
+
+		cache_seg_ctrl_init(cache_seg);
+
+		cache_seg->info_index = 0;
+		cache_seg_info_write(cache_seg);
+
+		/* clear outdated kset in segment */
+		memcpy_flushcache(segment->data, &pcache_empty_kset, sizeof(struct pcache_cache_kset_onmedia));
+		pmem_wmb();
+	} else {
+		ret = cache_seg_meta_load(cache_seg);
+		if (ret)
+			goto err;
+	}
+
+	return 0;
+err:
+	return ret;
+}
+
+/**
+ * get_cache_segment - Retrieves a free cache segment from the cache.
+ * @cache: Pointer to the cache structure.
+ *
+ * This function attempts to find a free cache segment that can be used.
+ * It locks the segment map and checks for the next available segment ID.
+ * If a free segment is found, it initializes it and returns a pointer to the
+ * cache segment structure. Returns NULL if no segments are available.
+ */
+struct pcache_cache_segment *get_cache_segment(struct pcache_cache *cache)
+{
+	struct pcache_cache_segment *cache_seg;
+	u32 seg_id;
+
+	spin_lock(&cache->seg_map_lock);
+again:
+	seg_id = find_next_zero_bit(cache->seg_map, cache->n_segs, cache->last_cache_seg);
+	if (seg_id == cache->n_segs) {
+		/* reset the hint of ->last_cache_seg and retry */
+		if (cache->last_cache_seg) {
+			cache->last_cache_seg = 0;
+			goto again;
+		}
+		cache->cache_full = true;
+		spin_unlock(&cache->seg_map_lock);
+		return NULL;
+	}
+
+	/*
+	 * found an available cache_seg, mark it used in seg_map
+	 * and update the search hint ->last_cache_seg
+	 */
+	__set_bit(seg_id, cache->seg_map);
+	cache->last_cache_seg = seg_id;
+	spin_unlock(&cache->seg_map_lock);
+
+	cache_seg = &cache->segments[seg_id];
+	cache_seg->cache_seg_id = seg_id;
+
+	return cache_seg;
+}
+
+static void cache_seg_gen_increase(struct pcache_cache_segment *cache_seg)
+{
+	spin_lock(&cache_seg->gen_lock);
+	cache_seg->gen++;
+	spin_unlock(&cache_seg->gen_lock);
+
+	cache_seg_ctrl_write(cache_seg);
+}
+
+void cache_seg_get(struct pcache_cache_segment *cache_seg)
+{
+	atomic_inc(&cache_seg->refs);
+}
+
+static void cache_seg_invalidate(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_cache *cache;
+
+	cache = cache_seg->cache;
+	cache_seg_gen_increase(cache_seg);
+
+	spin_lock(&cache->seg_map_lock);
+	if (cache->cache_full)
+		cache->cache_full = false;
+	__clear_bit(cache_seg->cache_seg_id, cache->seg_map);
+	spin_unlock(&cache->seg_map_lock);
+
+	pcache_defer_reqs_kick(CACHE_TO_PCACHE(cache));
+	/* clean_work will clean the bad key in key_tree*/
+	queue_work(cache_get_wq(cache), &cache->clean_work);
+}
+
+void cache_seg_put(struct pcache_cache_segment *cache_seg)
+{
+	if (atomic_dec_and_test(&cache_seg->refs))
+		cache_seg_invalidate(cache_seg);
+}