Messages in this thread | | | Date | Mon, 18 Jan 2021 10:41:16 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v3 4/4] arm64: mte: Optimize mte_assign_mem_tag_range() |
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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().
Thanks, Mark.
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