On 1/18/21 10:41 AM, Mark Rutland wrote:

On Sun, Jan 17, 2021 at 12:27:08PM +0000, Vincenzo Frascino wrote:

quoted

Hi Mark,

On 1/16/21 2:22 PM, Vincenzo Frascino wrote:

quoted

quoted

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

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