On 1/18/21 10:41 AM, Mark Rutland wrote:
> On Sun, Jan 17, 2021 at 12:27:08PM +0000, Vincenzo Frascino wrote:
>> Hi Mark,
>>
>> On 1/16/21 2:22 PM, Vincenzo Frascino wrote:
>>>> Is there any chance that this can be used for the last bytes of the
>>>> virtual address space? This might need to change to `_addr == _end` if
>>>> that is possible, otherwise it'll terminate early in that case.
>>>>
>>> Theoretically it is a possibility. I will change the condition and add a
>>> note
>>> for that.
>>>
>>
>> I was thinking to the end of the virtual address space scenario and I forgot
>> that if I use a condition like `_addr == _end` the tagging operation
>> overflows
>> to the first granule of the next allocation. This disrupts tagging accesses
>> for
>> that memory area hence I think that `_addr < _end` is the way to go.
>
> I think it implies `_addr != _end` is necessary. Otherwise, if `addr` is
> PAGE_SIZE from the end of memory, and `size` is PAGE_SIZE, `_end` will
> be 0, so using `_addr < _end` will mean the loop will terminate after a
> single MTE tag granule rather than the whole page.
>
> Generally, for some addr/increment/size combination (where all are
> suitably aligned), you need a pattern like:
>
> | do {
> | thing(addr);
> | addr += increment;
> | } while (addr != end);
>
> ... or:
>
> | for (addr = start; addr != end; addr += increment) {
> | thing(addr);
> | }
>
> ... to correctly handle working at the very end of the VA space.
>
> We do similar for page tables, e.g. when we use pmd_addr_end().
>
Good point! I agree it wraps around otherwise. I will change it accordingly.
Thanks!
> Thanks,
> Mark.
>
--
Regards,
Vincenzo