Re: [RFC PATCH 1/8] mm/madvise: propagate vma->vm_end changes

[Kirill A. Shutemov]

On Mon, Sep 27, 2021 at 05:33:39AM -0700, Nadav Amit wrote:
> 
> 
> > On Sep 27, 2021, at 4:55 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> > 
> > On Mon, Sep 27, 2021 at 03:11:20AM -0700, Nadav Amit wrote:
> >> 
> >>> On Sep 27, 2021, at 2:08 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> >>> 
> >>> On Sun, Sep 26, 2021 at 09:12:52AM -0700, Nadav Amit wrote:
> >>>> From: Nadav Amit <namit@xxxxxxxxxx>
> >>>> 
> >>>> The comment in madvise_dontneed_free() says that vma splits that occur
> >>>> while the mmap-lock is dropped, during userfaultfd_remove(), should be
> >>>> handled correctly, but nothing in the code indicates that it is so: prev
> >>>> is invalidated, and do_madvise() will therefore continue to update VMAs
> >>>> from the "obsolete" end (i.e., the one before the split).
> >>>> 
> >>>> Propagate the changes to end from madvise_dontneed_free() back to
> >>>> do_madvise() and continue the updates from the new end accordingly.
> >>> 
> >>> Could you describe in details a race that would lead to wrong behaviour?
> >> 
> >> Thanks for the quick response.
> >> 
> >> For instance, madvise(MADV_DONTNEED) can race with mprotect() and cause
> >> the VMA to split.
> >> 
> >> Something like:
> >> 
> >>  CPU0				CPU1
> >>  ----				----
> >>  madvise(0x10000, 0x2000, MADV_DONTNEED)
> >>  -> userfaultfd_remove()
> >>   [ mmap-lock dropped ]
> >> 				mprotect(0x11000, 0x1000, PROT_READ)
> >> 				[splitting the VMA]
> >> 
> >> 				read(uffd)
> >> 				[unblocking userfaultfd_remove()]
> >> 
> >>   [ resuming ]
> >>   end = vma->vm_end
> >>   [end == 0x11000]
> >> 
> >>   madvise_dontneed_single_vma(vma, 0x10000, 0x11000)
> >> 
> >>  Following this operation, 0x11000-0x12000 would not be zapped.
> > 
> > Okay, fair enough.
> > 
> > Wouldn't something like this work too:
> > 
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 0734db8d53a7..0898120c5c04 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -796,6 +796,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
> > 			 */
> > 			return -ENOMEM;
> > 		}
> > +		*prev = vma;
> > 		if (!can_madv_lru_vma(vma))
> > 			return -EINVAL;
> > 		if (end > vma->vm_end) {
> 
> Admittedly (embarrassingly?) I didn’t even consider it since all the
> comments say that once the lock is dropped prev should be invalidated.
> 
> Let’s see, considering the aforementioned scenario and that there is
> initially one VMA between 0x10000-0x12000.
> 
> Looking at the code from do_madvise()):
> 
> [ end == 0x12000 ]
> 
>                 tmp = vma->vm_end;
> 
> [ tmp == 0x12000 ]
> 
>                 if (end < tmp)
>                         tmp = end;
> 
>                 /* Here vma->vm_start <= start < tmp <= (end|vma->vm_end). */
> 
>                 error = madvise_vma(vma, &prev, start, tmp, behavior);
> 
> [ prev->vm_end == 0x11000 after the split]
> 
>                 if (error)
>                         goto out;
>                 start = tmp;
> 
> [ start == 0x12000 ]
> 
>                 if (prev && start < prev->vm_end)
>                         start = prev->vm_end;
> 
> [ The condition (start < prev->vm_end) is false, start not updated ]
> 
>                 error = unmapped_error;
>                 if (start >= end)
>                         goto out;
> 
> [ start >= end; so we end without updating the second part of the split ]
> 
> So it does not work.
> 
> Perhaps adding this one on top of yours? I can test it when I wake up.
> It is cleaner, but I am not sure if I am missing something.

It should work.

BTW, shouldn't we bring madvise_willneed() and madvise_remove() to the
same scheme?

-- 
 Kirill A. Shutemov