Re: [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes

Peter Zijlstra
2021-10-25T10:50:49Z

On Thu, Oct 21, 2021 at 08:04:50PM -0700, Andrew Morton wrote:
> On Thu, 21 Oct 2021 05:21:07 -0700 Nadav Amit <nadav.amit@gmail.com> wrote:
>
> > This patch-set is intended to remove unnecessary TLB flushes. It is
> > based on feedback from v1 and several bugs I found in v1 myself.
> >
> > Basically, there are 3 optimizations in this patch-set:
> > 1. Avoiding TLB flushes on change_huge_pmd() that are only needed to
> > prevent the A/D bits from changing.
> > 2. Use TLB batching infrastructure to batch flushes across VMAs and
> > do better/fewer flushes.
> > 3. Avoid TLB flushes on permission demotion.
> >
> > Andrea asked for the aforementioned (2) to come after (3), but this
> > is not simple (specifically since change_prot_numa() needs the number
> > of pages affected).
>
> [1/5] appears to be a significant fix which should probably be
> backported into -stable kernels. If you agree with this then I suggest
> it be prepared as a standalone patch, separate from the other four
> patches. With a cc:stable.

I am confused, 1/5 doesn't actually do *anything*. I also cannot find
any further usage of the introduced X86_BUG_PTE_LEAK.

I'm thinking patch #2 means to have something like:

if (cpu_feature_enabled(X86_BUG_PTE_LEAK))
flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);

In the newly minted: pmdp_invalidate_ad(), but alas, nothing there.