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

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>