kernel/linux.git/drivers/md/dm-pcache/cache_segment.c, branch v6.19.11

dm pcache: fix segment info indexing

2025-12-10T18:28:23+00:00

Segment info indexing also used sizeof(struct) instead of the 4K metadata stride, so info_index could point between slots and subsequent writes would advance incorrectly. Derive info_index from the pointer returned by the segment meta search using PCACHE_SEG_INFO_SIZE and advance to the next slot for future updates. Signed-off-by: Li Chen Signed-off-by: Dongsheng Yang Signed-off-by: Mikulas Patocka Reviewed-by: Zheng Gu Cc: stable@vger.kernel.org # 6.18

dm-pcache: advance slot index before writing slot

2025-12-10T18:28:23+00:00

In dm-pcache, in order to ensure crash-consistency, a dual-copy scheme is used to alternately update metadata, and there is a slot index that records the current slot. However, in the write path the current implementation writes directly to the current slot indexed by slot index, and then advances the slot — which ends up overwriting the existing slot, violating the crash-consistency guarantee. This patch fixes that behavior, preventing metadata from being overwritten incorrectly. In addition, this patch add a missing pmem_wmb() after memcpy_flushcache(). Signed-off-by: Dongsheng Yang Signed-off-by: Mikulas Patocka Reviewed-by: Zheng Gu Cc: stable@vger.kernel.org # 6.18

dm-pcache: remove ctrl_lock for pcache_cache_segment

2025-09-01T11:30:39+00:00

The smatch checker reports a “scheduler in atomic context” problem in the following call chain: miss_read_end_req() -> cache_seg_put() -> cache_seg_invalidate() -> cache_seg_gen_increase() -> mutex_lock(&cache_seg->ctrl_lock); In practice, this `mutex_lock` will not actually schedule, because it is only called when `cache_seg_put()` drops the last reference, which is single-threaded. That is also why the issue never shows up during real testing. However, the code is still buggy. The original purpose of `ctrl_lock` was to prevent read/write conflicts on the cache segment control information. Looking at the current usage, all control information accesses are single-threaded: reads only occur during the init phase, where no conflicts are possible, and writes happen once in the init phase (also single-threaded) and once when `cache_seg_put()` drops the last reference (again single-threaded). Therefore, this patch removes `ctrl_lock` entirely and adds comments in the appropriate places to document this logic. Signed-off-by: Dongsheng Yang Signed-off-by: Mikulas Patocka

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

2025-08-25T13:25:29+00:00

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 Signed-off-by: Mikulas Patocka