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Re: [dm-devel] dm cache metadata: set dirty on all cache blocks after a crash



On Thu, Aug 09 2018 at 12:22pm -0400,
Ilya Dryomov <idryomov gmail com> wrote:

> Quoting Documentation/device-mapper/cache.txt:
> 
>   The 'dirty' state for a cache block changes far too frequently for us
>   to keep updating it on the fly.  So we treat it as a hint.  In normal
>   operation it will be written when the dm device is suspended.  If the
>   system crashes all cache blocks will be assumed dirty when restarted.
> 
> This got broken in commit f177940a8091 ("dm cache metadata: switch to
> using the new cursor api for loading metadata") in 4.9, which removed
> the code that consulted cmd->clean_when_opened (CLEAN_SHUTDOWN on-disk
> flag) when loading cache blocks.  This results in data corruption on an
> unclean shutdown with dirty cache blocks on the fast device.  After the
> crash those blocks are considired clean and may get evicted from the
> cache at any time.  This can be demonstrated by doing a lot of reads
> to trigger individual evictions, but uncache is more predictable:
> 
>   ### Disable auto-activation in lvm.conf to be able to do uncache in
>   ### time (i.e. see uncache doing flushing) when the fix is applied.
> 
>   # xfs_io -d -c 'pwrite -b 4M -S 0xaa 0 1G' /dev/vdb
>   # vgcreate vg_cache /dev/vdb /dev/vdc
>   # lvcreate -L 1G -n lv_slowdev vg_cache /dev/vdb
>   # lvcreate -L 512M -n lv_cachedev vg_cache /dev/vdc
>   # lvcreate -L 256M -n lv_metadev vg_cache /dev/vdc
>   # lvconvert --type cache-pool --cachemode writeback vg_cache/lv_cachedev --poolmetadata vg_cache/lv_metadev
>   # lvconvert --type cache vg_cache/lv_slowdev --cachepool vg_cache/lv_cachedev
>   # xfs_io -d -c 'pwrite -b 4M -S 0xbb 0 512M' /dev/mapper/vg_cache-lv_slowdev
>   # xfs_io -d -c 'pread -v 254M 512' /dev/mapper/vg_cache-lv_slowdev | head -n 2
>   0fe00000:  bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
>   0fe00010:  bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
>   # dmsetup status vg_cache-lv_slowdev
>   0 2097152 cache 8 27/65536 128 8192/8192 1 100 0 0 0 8192 7065 2 metadata2 writeback 2 migration_threshold 2048 smq 0 rw -
>                                                             ^^^^
>                                 7065 * 64k = 441M yet to be written to the slow device
>   # echo b >/proc/sysrq-trigger
> 
>   # vgchange -ay vg_cache
>   # xfs_io -d -c 'pread -v 254M 512' /dev/mapper/vg_cache-lv_slowdev | head -n 2
>   0fe00000:  bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
>   0fe00010:  bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
>   # lvconvert --uncache vg_cache/lv_slowdev
>   Flushing 0 blocks for cache vg_cache/lv_slowdev.
>   Logical volume "lv_cachedev" successfully removed
>   Logical volume vg_cache/lv_slowdev is not cached.
>   # xfs_io -d -c 'pread -v 254M 512' /dev/mapper/vg_cache-lv_slowdev | head -n 2
>   0fe00000:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa  ................
>   0fe00010:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa  ................
> 
> This is the case with both v1 and v2 cache pool metatata formats.
> 
> After applying this patch:
> 
>   # vgchange -ay vg_cache
>   # xfs_io -d -c 'pread -v 254M 512' /dev/mapper/vg_cache-lv_slowdev | head -n 2
>   0fe00000:  bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
>   0fe00010:  bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
>   # lvconvert --uncache vg_cache/lv_slowdev
>   Flushing 3724 blocks for cache vg_cache/lv_slowdev.
>   ...
>   Flushing 71 blocks for cache vg_cache/lv_slowdev.
>   Logical volume "lv_cachedev" successfully removed
>   Logical volume vg_cache/lv_slowdev is not cached.
>   # xfs_io -d -c 'pread -v 254M 512' /dev/mapper/vg_cache-lv_slowdev | head -n 2
>   0fe00000:  bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
>   0fe00010:  bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
> 
> Cc: stable vger kernel org
> Fixes: f177940a8091 ("dm cache metadata: switch to using the new cursor api for loading metadata")
> Signed-off-by: Ilya Dryomov <idryomov gmail com>

I staged this earlier today for 4.19 inclusion, please see:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.19&id=5b1fe7bec8a8d0cc547a22e7ddc2bd59acd67de4

Thanks so much for your thorough work on this.  Very well done!
Mike


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