linux-kernel - Re: [PATCH 0/2] dm-pcache: fix metadata indexing and persistence issues

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Message-ID: <7a233190-82ae-b95c-1988-9b746ef6a8b1@redhat.com>
Date: Tue, 2 Dec 2025 20:09:07 +0100 (CET)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Li Chen <me@...ux.beauty>
cc: dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org, 
    Dongsheng Yang <dongsheng.yang@...ux.dev>, Zheng Gu <cengku@...il.com>
Subject: Re: [PATCH 0/2] dm-pcache: fix metadata indexing and persistence
 issues

Both patches accepted, thanks.

Mikulas



On Tue, 2 Dec 2025, Li Chen wrote:

> From: Li Chen <chenl311@...natelecom.cn>
> 
> dm-pcache stores metadata (cache_info and segment_info) in 4K-aligned
> slots on the cache device, using sequence numbers and CRC to identify
> the latest valid copy.
> 
> However, the cache_info and segment_info paths were computing their
> on-media index using sizeof(struct) instead of the 4K metadata stride.
> As a result:
> 
>   * cache_info updates (including gc_percent set via a dmsetup message)
>     were written to invalid offsets between metadata slots and failed
>     to persist across table reloads or reboots.
> 
>   * segment_info indexing became desynchronized, so rotation to the
>     "next" slot no longer matched the location returned by
>     pcache_meta_find_latest().
> 
> The issue can be reproduced with:
> 
>   dmsetup create pcache_vdb --table \
>     "0 $(blockdev --getsz /dev/vdb) pcache /dev/pmem0 /dev/vdb 4 \
>      cache_mode writeback data_crc false"
> 
>   # Check default gc_percent (70)
>   dmsetup status pcache_vdb
> 
>   # Change gc_percent to 10
>   dmsetup message pcache_vdb 0 "gc_percent 10"
> 
>   # Verify change is active in memory
>   dmsetup status pcache_vdb
> 
>   # Reboot the system...
> 
>   # Without patch (gc_percent reverts to 70):
>   dmsetup status pcache_vdb
> 
>   # With patch (gc_percent persists as 10):
>   dmsetup status pcache_vdb
> 
> This series fixes the issue by deriving the metadata slot index from
> the pointer returned by pcache_meta_find_latest(), using the 4K stride
> (CACHE_INFO_SIZE / SEG_INFO_SIZE). This ensures that updates to
> cache_info and segment_info are written to valid slots and remain
> consistent with the on-media layout.
> 
> Li Chen (2):
>   dm pcache: fix cache info indexing
>   dm pcache: fix segment info indexing
> 
>  drivers/md/dm-pcache/cache.c         | 13 ++++++++++---
>  drivers/md/dm-pcache/cache_segment.c |  6 +++++-
>  2 files changed, 15 insertions(+), 4 deletions(-)
> 
> -- 
> 2.51.0
> 
>