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

[Andy Lutomirski]

> On Jan 12, 2021, at 2:29 PM, Nadav Amit <nadav.amit@xxxxxxxxx> 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.

If you want a hand, let me know.  I suspect you understand the x86 code as well as I do at this point, though :)