[dm-devel] [git pull] device mapper changes for 4.21

[Milan Broz]

On 03/01/2019 08:27, Christoph Hellwig wrote:
> On Sun, Dec 30, 2018 at 04:25:46PM -0800, Linus Torvalds wrote:
>> On Sun, Dec 30, 2018 at 11:15 AM Mikulas Patocka <mpatocka at redhat.com> wrote:
>>>
>>> But you're right that 2TiB devices are common and that perhaps this option
>>> should go away.
>>
>> 2TiB devices are definitely not common in the one situation where this
>> option might matter: small embedded devices.
>>
>> I don't think the cost of 64 bit is in the arithmetic, but it might be
>> in some of the data structures.
>>
>> But my gut feel is that it probably doesn't much matter, and we could
>> get rid of the config option without anybody ever noticing. I don't
>> think we have that many data structures with 'sector_t' in them.
>>
>> We might try to first just force the option on, and see if anybody even cares.
> 
> Our smallest embedded devices use raw flash using the MTD subsystem,
> and even that is using 64-bit size types everywhere.  So I'd be really
> surprised if it is an issue.

I agree with Christoph here.

(I fixed some CONFIG_LBDAF problems in DM in this pull request because
the code was apparently wrong, but it was a pain to see all these possible
sector overflow checks...)

If it helps anything, we require 64-bit calculations for cryptsetup userspace for >5 years
(you cannot compile it with 32bit support; everything uses 64bit, including these
DM table sector calculations for kernel) and NOBODY complained since we enforced it.
(Ok, it is not a hot path, but....)

Please, if possible, go with 64-bit sector size types by default in future.

Thanks,
Milan