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[dm-devel] Re: [PATCH] dm snapshot: don't use zero chunksize



On Thu, 8 Oct 2009, Alasdair G Kergon wrote:

> On Tue, Oct 06, 2009 at 07:01:37PM -0400, Mikulas Patocka wrote:
> > Under some special conditions (too big chunk size or zero-sized device),
> > the resulting hash_size is calculated as zero.
> > 
> > rounddown_pow_of_two(0) is undefined operation (it expands to shift by -1).
> > And init_exception_table with zero argument would fail with -ENOMEM.
> > 
> > This patch makes minimum chunk size 64, just like for pending exception table.
>  
> Could we have some more specific information in this patch header?
> 
> How does a "too big" chunk size happen?  And what is "too big"?

The problem is if you have bigger chunk size than the origin device (this 
is valid, but useless). Then, hash_size is set to zero and false 
allocation failure is reported.

I think bug 502965 is caused by this --- Milan tried to create a 10k 
volume with 16k chunksize.

> How does a zero-sized device happen and how can this code do anything
> sensible if it encounters one - shouldn't it fail with an error instead?
> Or do we still have a problem with the sequence in which dm ioctls are being
> issued?

I don't know if zero-sized device can happen with lvm. I think not. It 
could be only created manually with dmsetup. Snapshot of a zero-sized 
device may be still considered allowed.

> Alasdairs

Mikulas


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