Re: [PATCH v3 4/4] arm64: mte: Optimize mte_assign_mem_tag_range()

From: Mark Rutland
Date: Mon Jan 18 2021 - 15:29:52 EST

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().