Introduce cache_gc.c, a self-contained engine that reclaims cache
segments whose data have already been flushed to the backing device.
Running in the cache workqueue, the GC keeps segment usage below the
user-configurable *cache_gc_percent* threshold.
* need_gc() – decides when to trigger GC by checking:
- *dirty_tail* vs *key_tail* position,
- kset integrity (magic + CRC),
- bitmap utilisation against the gc-percent threshold.
* Per-key reclamation
- Decodes each key in the target kset (`cache_key_decode()`).
- Drops the segment reference with `cache_seg_put()`, allowing the
segment to be invalidated once all keys are gone.
- When the reference count hits zero the segment is cleared from
`seg_map`, making it immediately reusable by the allocator.
* Scheduling
- `pcache_cache_gc_fn()` loops until no more work is needed, then
re-queues itself after *PCACHE_CACHE_GC_INTERVAL*.
Signed-off-by: Dongsheng Yang <dongsheng.y...@linux.dev>
---
drivers/md/dm-pcache/cache_gc.c | 170 ++++++++++++++++++++++++++++++++
1 file changed, 170 insertions(+)
create mode 100644 drivers/md/dm-pcache/cache_gc.c
diff --git a/drivers/md/dm-pcache/cache_gc.c b/drivers/md/dm-pcache/cache_gc.c
new file mode 100644
index 000000000000..a122d95c8f46
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_gc.c
@@ -0,0 +1,170 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include "cache.h"
+#include "backing_dev.h"
+#include "cache_dev.h"
+#include "dm_pcache.h"
+
+/**
+ * cache_key_gc - Releases the reference of a cache key segment.
+ * @cache: Pointer to the pcache_cache structure.
+ * @key: Pointer to the cache key to be garbage collected.
+ *
+ * This function decrements the reference count of the cache segment
+ * associated with the given key. If the reference count drops to zero,
+ * the segment may be invalidated and reused.
+ */
+static void cache_key_gc(struct pcache_cache *cache, struct pcache_cache_key
*key)
+{
+ cache_seg_put(key->cache_pos.cache_seg);
+}
+
+static bool need_gc(struct pcache_cache *cache, struct pcache_cache_pos
*dirty_tail, struct pcache_cache_pos *key_tail)
+{
+ struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+ struct pcache_cache_kset_onmedia *kset_onmedia;
+ void *dirty_addr, *key_addr;
+ u32 segs_used, segs_gc_threshold, to_copy;
+ int ret;
+
+ dirty_addr = cache_pos_addr(dirty_tail);
+ key_addr = cache_pos_addr(key_tail);
+ if (dirty_addr == key_addr) {
+ pcache_dev_debug(pcache, "key tail is equal to dirty tail:
%u:%u\n",
+ dirty_tail->cache_seg->cache_seg_id,
+ dirty_tail->seg_off);
+ return false;
+ }
+
+ kset_onmedia = (struct pcache_cache_kset_onmedia
*)cache->gc_kset_onmedia_buf;
+
+ to_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, PCACHE_SEG_SIZE -
key_tail->seg_off);
+ ret = copy_mc_to_kernel(kset_onmedia, key_addr, to_copy);
+ if (ret) {
+ pcache_dev_err(pcache, "error to read kset: %d", ret);
+ return false;
+ }
+
+ /* Check if kset_onmedia is corrupted */
+ if (kset_onmedia->magic != PCACHE_KSET_MAGIC) {
+ pcache_dev_debug(pcache, "gc error: magic is not as expected.
key_tail: %u:%u magic: %llx, expected: %llx\n",
+ key_tail->cache_seg->cache_seg_id,
key_tail->seg_off,
+ kset_onmedia->magic, PCACHE_KSET_MAGIC);
+ return false;
+ }
+
+ /* Verify the CRC of the kset_onmedia */
+ if (kset_onmedia->crc != cache_kset_crc(kset_onmedia)) {
+ pcache_dev_debug(pcache, "gc error: crc is not as expected.
crc: %x, expected: %x\n",
+ cache_kset_crc(kset_onmedia),
kset_onmedia->crc);
+ return false;
+ }
+
+ segs_used = bitmap_weight(cache->seg_map, cache->n_segs);
+ segs_gc_threshold = cache->n_segs * pcache_cache_get_gc_percent(cache)
/ 100;
+ if (segs_used < segs_gc_threshold) {
+ pcache_dev_debug(pcache, "segs_used: %u, segs_gc_threshold:
%u\n", segs_used, segs_gc_threshold);
+ return false;
+ }
+
+ return true;
+}
+
+/**
+ * last_kset_gc - Advances the garbage collection for the last kset.
+ * @cache: Pointer to the pcache_cache structure.
+ * @kset_onmedia: Pointer to the kset_onmedia structure for the last kset.
+ */
+static void last_kset_gc(struct pcache_cache *cache, struct
pcache_cache_kset_onmedia *kset_onmedia)
+{
+ struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+ struct pcache_cache_segment *cur_seg, *next_seg;
+
+ cur_seg = cache->key_tail.cache_seg;
+
+ next_seg = &cache->segments[kset_onmedia->next_cache_seg_id];
+
+ mutex_lock(&cache->key_tail_lock);
+ cache->key_tail.cache_seg = next_seg;
+ cache->key_tail.seg_off = 0;
+ cache_encode_key_tail(cache);
+ mutex_unlock(&cache->key_tail_lock);
+
+ pcache_dev_debug(pcache, "gc advance kset seg: %u\n",
cur_seg->cache_seg_id);
+
+ spin_lock(&cache->seg_map_lock);
+ clear_bit(cur_seg->cache_seg_id, cache->seg_map);
+ spin_unlock(&cache->seg_map_lock);
+}
+
+void pcache_cache_gc_fn(struct work_struct *work)
+{
+ struct pcache_cache *cache = container_of(work, struct pcache_cache,
gc_work.work);
+ struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+ struct pcache_cache_pos dirty_tail, key_tail;
+ struct pcache_cache_kset_onmedia *kset_onmedia;
+ struct pcache_cache_key_onmedia *key_onmedia;
+ struct pcache_cache_key *key;
+ int ret;
+ int i;
+
+ kset_onmedia = (struct pcache_cache_kset_onmedia
*)cache->gc_kset_onmedia_buf;
+
+ while (true) {
+ if (pcache_is_stopping(pcache) ||
atomic_read(&cache->gc_errors))
+ return;
+
+ /* Get new tail positions */
+ mutex_lock(&cache->dirty_tail_lock);
+ cache_pos_copy(&dirty_tail, &cache->dirty_tail);
+ mutex_unlock(&cache->dirty_tail_lock);
+
+ mutex_lock(&cache->key_tail_lock);
+ cache_pos_copy(&key_tail, &cache->key_tail);
+ mutex_unlock(&cache->key_tail_lock);
+
+ if (!need_gc(cache, &dirty_tail, &key_tail))
+ break;
+
+ if (kset_onmedia->flags & PCACHE_KSET_FLAGS_LAST) {
+ /* Don't move to the next segment if dirty_tail has not
moved */
+ if (dirty_tail.cache_seg == key_tail.cache_seg)
+ break;
+
+ last_kset_gc(cache, kset_onmedia);
+ continue;
+ }
+
+ for (i = 0; i < kset_onmedia->key_num; i++) {
+ struct pcache_cache_key key_tmp = { 0 };
+
+ key_onmedia = &kset_onmedia->data[i];
+
+ key = &key_tmp;
+ cache_key_init(&cache->req_key_tree, key);
+
+ ret = cache_key_decode(cache, key_onmedia, key);
+ if (ret) {
+ /* return without re-arm gc work, and prevent
future
+ * gc, because we can't retry the partial-gc-ed
kset
+ */
+ atomic_inc(&cache->gc_errors);
+ pcache_dev_err(pcache, "failed to decode cache
key in gc\n");
+ return;
+ }
+
+ cache_key_gc(cache, key);
+ }
+
+ pcache_dev_debug(pcache, "gc advance: %u:%u %u\n",
+ key_tail.cache_seg->cache_seg_id,
+ key_tail.seg_off,
+ get_kset_onmedia_size(kset_onmedia));
+
+ mutex_lock(&cache->key_tail_lock);
+ cache_pos_advance(&cache->key_tail,
get_kset_onmedia_size(kset_onmedia));
+ cache_encode_key_tail(cache);
+ mutex_unlock(&cache->key_tail_lock);
+ }
+
+ queue_delayed_work(cache_get_wq(cache), &cache->gc_work,
PCACHE_CACHE_GC_INTERVAL);
+}
--
2.43.0
