Re: [PATCH 1/2] mm/uffd: Fix vma operation where start addr cuts part of vma

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Wed, May 17, 2023 at 02:54:39PM -0400, Peter Xu wrote:
> On Wed, May 17, 2023 at 07:40:59PM +0100, Lorenzo Stoakes wrote:
> > On Wed, May 17, 2023 at 02:37:41PM -0400, Peter Xu wrote:
> > > On Wed, May 17, 2023 at 06:20:55PM +0100, Lorenzo Stoakes wrote:
> > > > On Wed, May 17, 2023 at 11:04:07AM -0400, Peter Xu wrote:
> > > > > It seems vma merging with uffd paths is broken with either
> > > > > register/unregister, where right now we can feed wrong parameters to
> > > > > vma_merge() and it's found by recent patch which moved asserts upwards in
> > > > > vma_merge() by Lorenzo Stoakes:
> > > > >
> > > > > https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/
> > > > >
> > > > > The problem is in the current code base we didn't fixup "prev" for the case
> > > > > where "start" address can be within the "prev" vma section.  In that case
> > > > > we should have "prev" points to the current vma rather than the previous
> > > > > one when feeding to vma_merge().
> > > >
> > > > This doesn't seem quite correct, perhaps - "where start is contained within vma
> > > > but not clamped to its start. We need to convert this into case 4 which permits
> > > > subdivision of prev by assigning vma to prev. As we loop, each subsequent VMA
> > > > will be clamped to the start."
> > >
> > > I think it covers more than case 4 - it can also be case 0 where no merge
> > > will happen?
> >
> > Ugh please let's not call a case that doesn't merge by a number :P but sure of
> > course it might also not merge.
>
> To me the original paragraph was still fine. But if you prefer your version
> (which I'm perfectly fine either way if you'd like to spell out what cases
> it'll trigger), it'll be:
>
>   It's possible that "start" is contained within vma but not clamped to its
>   start.  We need to convert this into either "cannot merge" case or "can
>   merge" case 4 which permits subdivision of prev by assigning vma to
>   prev. As we loop, each subsequent VMA will be clamped to the start.
>
> Does that look good to you?
>

Looks good to me, thanks for taking the time!

> Thanks,
>
> --
> Peter Xu
>