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

[Will Deacon]

On Tue, Jan 12, 2021 at 02:29:51PM -0800, Nadav Amit wrote:
> > On Jan 12, 2021, at 1:43 PM, Will Deacon <will@xxxxxxxxxx> 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.
> 
> Let me first finish making something that we can use as a basis for a
> discussion. I do not waste your time before I have something ready.

Sure thing! Give me a shout when you're ready.

Will