[patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault

[Nicholas Piggin]

Excerpts from Linus Torvalds's message of July 28, 2020 4:37 am:
> [ Adding linux-arch, just to make other architectures aware of this issue too.
> 
>   We have a "flush_tlb_fix_spurious_fault()" thing to take care of the
> "TLB may contain stale entries, we can't take the same fault over and
> over again" situation.
> 
>   On x86, it's a no-op, because x86 doesn't do that. x86 will re-walk
> the page tables - or possibly just always invalidate the faulting TLB
> entry - before taking a fault, so there can be no long-term stale
> TLB's.

[snip]

>   It looks like powerpc people at least thought about this, and only
> do it if there is a coprocessor. Which sounds a bit confused, but I
> don't know the rules.

I'm not sure about ppc32 and 64e, I'm almost certain they should do a 
local flush if anyting, and someone with a good understanding of the 
ISAs and CPUs might be able to nop it entirely. I agree global can't 
ever really make sense (except as a default because we have no generic 
local flush).

powerpc/64s reloads translations after taking a fault, so it's fine with 
a nop here.

The quirk is a problem with coprocessor where it's supposed to 
invalidate the translation after a fault but it doesn't, so we can get a 
read-only TLB stuck after something else does a RO->RW upgrade on the 
TLB. Something like that IIRC.  Coprocessors have their own MMU which 
lives in the nest not the core, so you need a global TLB flush to
invalidate that thing.

Thanks,
Nick