Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

[Yu Zhao]

On Sat, Jan 16, 2021 at 11:32:22PM -0800, Nadav Amit wrote:
> > On Jan 16, 2021, at 8:41 PM, Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> > 
> > On Tue, Jan 12, 2021 at 09:43:38PM +0000, Will Deacon wrote:
> >> On Tue, Jan 12, 2021 at 12:38:34PM -0800, Nadav Amit wrote:
> >>>> On Jan 12, 2021, at 11:56 AM, Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> >>>> On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote:
> >>>>> I will send an RFC soon for per-table deferred TLB flushes tracking.
> >>>>> The basic idea is to save a generation in the page-struct that tracks
> >>>>> when deferred PTE change took place, and track whenever a TLB flush
> >>>>> completed. In addition, other users - such as mprotect - would use
> >>>>> the tlb_gather interface.
> >>>>> 
> >>>>> Unfortunately, due to limited space in page-struct this would only
> >>>>> be possible for 64-bit (and my implementation is only for x86-64).
> >>>> 
> >>>> I don't want to discourage you but I don't think this would end up
> >>>> well. PPC doesn't necessarily follow one-page-struct-per-table rule,
> >>>> and I've run into problems with this before while trying to do
> >>>> something similar.
> >>> 
> >>> Discourage, discourage. Better now than later.
> >>> 
> >>> It will be relatively easy to extend the scheme to be per-VMA instead of
> >>> per-table for architectures that prefer it this way. It does require
> >>> TLB-generation tracking though, which Andy only implemented for x86, so I
> >>> will focus on x86-64 right now.
> >> 
> >> Can you remind me of what we're missing on arm64 in this area, please? I'm
> >> happy to help get this up and running once you have something I can build
> >> on.
> > 
> > I noticed arm/arm64 don't support ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH.
> > Would it be something worth pursuing? Arm has been using mm_cpumask,
> > so it might not be too difficult I guess?
> 
> [ +Mel Gorman who implemented ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH ]
> 
> IIUC, there are at least two bugs in x86 implementation.
> 
> First, there is a missing memory barrier in tlbbatch_add_mm() between
> inc_mm_tlb_gen() and the read of mm_cpumask().

In arch_tlbbatch_add_mm()? inc_mm_tlb_gen() has builtin barrier as its
comment says -- atomic update ops that return values are also full
memory barriers.

> Second, try_to_unmap_flush() clears flush_required after flushing. Another
> thread can call set_tlb_ubc_flush_pending() after the flush and before
> flush_required is cleared, and the indication that a TLB flush is pending
> can be lost.

This isn't a problem either because flush_required is per thread.

> I am working on addressing these issues among others, but, as you already
> saw, I am a bit slow.
> 
> On a different but related topic: Another thing that I noticed that Arm does
> not do is batching TLB flushes across VMAs. Since Arm does not have its own
> tlb_end_vma(), it uses the default tlb_end_vma(), which flushes each VMA
> separately. Peter Zijlstra’s comment says that there are advantages in
> flushing each VMA separately, but I am not sure it is better or intentional
> (especially since x86 does not do so).
> 
> I am trying to remove the arch-specific tlb_end_vma() and have a config
> option to control this behavior.

One thing worth noting is not all arm/arm64 hw versions support ranges.
(system_supports_tlb_range()). But IIUC what you are trying to do, this
isn't a problem.

> Again, sorry for being slow. I hope to send an RFC soon.

No worries. I brought it up only because I noticed it and didn't want
it to slip away.