Re: [RFC PATCH] mm: Fetch the dirty bit before we reset the pte

[Aneesh Kumar K.V]

On 10/8/20 10:32 PM, Linus Torvalds wrote:
> On Thu, Oct 8, 2020 at 2:27 AM Aneesh Kumar K.V
> <aneesh.kumar@xxxxxxxxxxxxx> wrote:
> > In copy_present_page, after we mark the pte non-writable, we should
> > check for previous dirty bit updates and make sure we don't lose the dirty
> > bit on reset.
>
> No, we'll just remove that entirely.
>
> Do you have a test-case that shows a problem? I have a patch that I
> was going to delay until 5.10 because I didn't think it mattered in
> practice..

Unfortunately, I don't have a test case. That was observed by code 
inspection while I was fixing syzkaller report.

> The second part of this patch would be to add a sequence count
> protection to fast-GUP pinning, so that GUP and fork() couldn't race,
> but I haven't written that part.
>
> Here's the first patch anyway. If you actually have a test-case where
> this matters, I guess I need to apply it now..
>
>                     Linus


-aneesh