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

[Yu Zhao]

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?