Re: 2.6.29-git13: Reported regressions from 2.6.28

Chris Friesen
2009-04-20T21:23:59Z

Linus Torvalds wrote:
>
> On Thu, 16 Apr 2009, Chris Friesen wrote:
>> I'm okay with that. The problem causes some backwards compatibility problems
>> with existing apps that get confused by the large "offset" number. The fix is
>> going to cause problems too, but in a different way.
>>
>> We'll work around it.
>
> If you have actual apps that care, that's a different issue.
>
> We do try to bend over backwards on ABI issues if it really is noticeable
> for applications. Now, in this case, if you can just fix your app to not
> care (because it really was badly written in the first place to even
> notice), then that is the _much_ superior solution.

Yep, we can fix the app to ignore that field for anonymous mappings.

> Although I don't really even see what we can sanely do except for the 0
> case. We could put the virtual address in there instead of zero (I forget
> what old kernels used to do - whatever magic value the anonymous mappings
> got, it wasn't really designed as an important value in its own right, it
> was designed to trigger the "we can merge these vma's" logic.

For anonymous mappings, the older kernels put the starting address of
the VMA (from the point of view of the app) as the offset. Until the
recent change, new kernels still did this for most VMAs, but the stack
offset was a 64-bit value with no obvious relation to the VMA start address.

Chris