Re: [PATCH v2 5/5] KVM: PPC: Book3S HV: radix do not clear partition scoped page table when page fault races with other vCPUs.

[Nicholas Piggin <npiggin@gmail.com>]

On Mon, 16 Apr 2018 14:32:40 +1000
Nicholas Piggin <npiggin@gmail.com> wrote:

> When running a SMP radix guest, KVM can get into page fault / tlbie
> storms -- hundreds of thousands to the same address from different
> threads -- due to partition scoped page faults invalidating the
> page table entry if it was found to be already set up by a racing
> CPU.
> 
> What can happen is that guest threads can hit page faults for the
> same addresses, this can happen when KSM or THP takes out a commonly
> used page. gRA zero (the interrupt vectors and important kernel text)
> was a common one. Multiple CPUs will page fault and contend on the
> same lock, when one CPU sets up the page table and releases the lock,
> the next will find the new entry and invalidate it before installing
> its own, which causes other page faults which invalidate that entry,
> etc.
> 
> The solution to this is to avoid invalidating the entry or flushing
> TLBs in case of a race. The pte may still need bits updated, but
> those are to add R/C or relax access restrictions so no flush is
> required.
> 
> This solves the page fault / tlbie storms.

Oh, I didn't notice "KVM: PPC: Book3S HV: Radix page fault handler
optimizations" does much the same thing as this one and it's been
merged upstream now.

That also adds a partition scoped PWC flush that I'll add to
powerpc/mm, so I'll rebase this series.

Thanks,
Nick