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Re: [dm-devel] [PATCH] block devices: validate block device capacity




On Mon, 3 Feb 2014, Christoph Hellwig wrote:

> On Fri, Jan 31, 2014 at 03:20:17AM -0500, Mikulas Patocka wrote:
> > So if you think you can support 16TiB devices and leave pgoff_t 32-bit, 
> > send a patch that does it.
> > 
> > Until you make it, you should apply the patch that I sent, that prevents 
> > kernel lockups or data corruption when the user uses 16TiB device on 
> > 32-bit kernel.
> 
> Exactly.  I had actually looked into support for > 16TiB devices for
> a NAS use case a while ago, but when explaining the effort involves
> the idea was dropped quickly.  The Linux block device is too deeply
> tied to the pagecache to make it easily feasible.

The memory management routines use pgoff_t, so we could define pgoff_t to 
be 64-bit type. But there is lib/radix_tree.c that uses unsigned long as 
an index into the radix tree - and pgoff_t is cast to unsigned long when 
calling the radix_tree routines - so we'd need to change lib/radix_tree to 
use pgoff_t.

Then, there may be other places where pgoff_t is cast to unsigned long and 
they are not trivial to find (one could enable some extra compiler 
warnings about truncating values when casting them, but I suppose, this 
would trigger a lot of false positives). This needs some deep review by 
people who designed the memory management code.

Mikulas


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