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[dm-devel] Strange mapped block count kept by dm-thin driver for thin devices



Hi,

I have already noticed that the mapped block count of thin devices
shown by "dmsetup status" looked unreasonable to me for a couple of
times in the last six months.
Sometimes the mapped block count was zero, but sometimes it was a
large number which was obviously larger than the total available
blocks in thin-pool.
So I dig into the source code, and I find some codes look suspicious to me.

In dm_pool_metadata_close() in dm-thin-metadata.c, it will try to
commit transaction for the last time before it destroys the
pmd(dm_pool_metadata).
However, it does not lock the pool_metadata before transaction commit,
which I am not really sure if we still need to acquire lock when it
reaches here.
I know that this function is mainly called when users try to remove
pool, but it seems like we can still have some on-the-fly transaction
commit when dm-thin module calls this function.
Although I would like to test it by myself, this problem occurs in
random and I am still trying to find a way to reproduce it
systematically.

Any help would be grateful.

Thanks

Dennis


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