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1d57628ff9
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>
118 lines
3.5 KiB
C
118 lines
3.5 KiB
C
/* SPDX-License-Identifier: GPL-2.0-or-later */
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#ifndef _PCACHE_INTERNAL_H
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#define _PCACHE_INTERNAL_H
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#include <linux/delay.h>
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#include <linux/crc32c.h>
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#define pcache_err(fmt, ...) \
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pr_err("dm-pcache: %s:%u " fmt, __func__, __LINE__, ##__VA_ARGS__)
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#define pcache_info(fmt, ...) \
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pr_info("dm-pcache: %s:%u " fmt, __func__, __LINE__, ##__VA_ARGS__)
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#define pcache_debug(fmt, ...) \
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pr_debug("dm-pcache: %s:%u " fmt, __func__, __LINE__, ##__VA_ARGS__)
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#define PCACHE_KB (1024ULL)
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#define PCACHE_MB (1024 * PCACHE_KB)
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/* Maximum number of metadata indices */
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#define PCACHE_META_INDEX_MAX 2
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#define PCACHE_CRC_SEED 0x3B15A
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/*
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* struct pcache_meta_header - PCACHE metadata header structure
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* @crc: CRC checksum for validating metadata integrity.
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* @seq: Sequence number to track metadata updates.
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* @version: Metadata version.
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* @res: Reserved space for future use.
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*/
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struct pcache_meta_header {
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__u32 crc;
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__u8 seq;
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__u8 version;
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__u16 res;
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};
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/*
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* pcache_meta_crc - Calculate CRC for the given metadata header.
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* @header: Pointer to the metadata header.
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* @meta_size: Size of the metadata structure.
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*
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* Returns the CRC checksum calculated by excluding the CRC field itself.
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*/
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static inline u32 pcache_meta_crc(struct pcache_meta_header *header, u32 meta_size)
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{
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return crc32c(PCACHE_CRC_SEED, (void *)header + 4, meta_size - 4);
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}
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/*
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* pcache_meta_seq_after - Check if a sequence number is more recent, accounting for overflow.
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* @seq1: First sequence number.
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* @seq2: Second sequence number.
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*
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* Determines if @seq1 is more recent than @seq2 by calculating the signed
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* difference between them. This approach allows handling sequence number
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* overflow correctly because the difference wraps naturally, and any value
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* greater than zero indicates that @seq1 is "after" @seq2. This method
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* assumes 8-bit unsigned sequence numbers, where the difference wraps
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* around if seq1 overflows past seq2.
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*
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* Returns:
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* - true if @seq1 is more recent than @seq2, indicating it comes "after"
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* - false otherwise.
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*/
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static inline bool pcache_meta_seq_after(u8 seq1, u8 seq2)
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{
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return (s8)(seq1 - seq2) > 0;
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}
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/*
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* pcache_meta_find_latest - Find the latest valid metadata.
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* @header: Pointer to the metadata header.
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* @meta_size: Size of each metadata block.
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*
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* Finds the latest valid metadata by checking sequence numbers. If a
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* valid entry with the highest sequence number is found, its pointer
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* is returned. Returns NULL if no valid metadata is found.
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*/
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static inline void __must_check *pcache_meta_find_latest(struct pcache_meta_header *header,
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u32 meta_size, u32 meta_max_size,
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void *meta_ret)
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{
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struct pcache_meta_header *meta, *latest = NULL;
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u32 i, seq_latest = 0;
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void *meta_addr;
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meta = meta_ret;
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for (i = 0; i < PCACHE_META_INDEX_MAX; i++) {
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meta_addr = (void *)header + (i * meta_max_size);
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if (copy_mc_to_kernel(meta, meta_addr, meta_size)) {
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pcache_err("hardware memory error when copy meta");
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return ERR_PTR(-EIO);
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}
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/* Skip if CRC check fails, which means corrupted */
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if (meta->crc != pcache_meta_crc(meta, meta_size))
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continue;
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/* Update latest if a more recent sequence is found */
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if (!latest || pcache_meta_seq_after(meta->seq, seq_latest)) {
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seq_latest = meta->seq;
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latest = (void *)header + (i * meta_max_size);
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}
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}
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if (!latest)
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return NULL;
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if (copy_mc_to_kernel(meta_ret, latest, meta_size)) {
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pcache_err("hardware memory error");
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return ERR_PTR(-EIO);
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}
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return latest;
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}
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#endif /* _PCACHE_INTERNAL_H */
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