Re: [PATCH v2] mm/mmap: return early if it can't merge in vma_merge()

[Lorenzo Stoakes]

On Thu, Feb 22, 2024 at 03:47:04PM +0800, Yajun Deng wrote:
>
> On 2024/2/22 04:41, Lorenzo Stoakes wrote:
> > On Wed, Feb 21, 2024 at 10:38:27AM -0500, Liam R. Howlett wrote:
> > > * Yajun Deng <yajun.deng@xxxxxxxxx> [240221 04:15]:
> > > > In most cases, the range of the area is valid. But in do_mprotect_pkey(),
> > > > the minimum value of end and vma->vm_end is passed to mprotect_fixup().
> > > > This will lead to the end is less than the end of prev.
> > > >
> > > > In this case, the curr will be NULL, but the next will be equal to the
> > > > prev. So it will attempt to merge before, the vm_pgoff check will cause
> > > > this case to fail.
> > > >
> > > > To avoid the process described above and reduce unnecessary operations.
> > > > Add a check to immediately return NULL if the end is less than the end of
> > > > prev.
> > > If it's only one caller, could we stop that caller instead of checking
> > > an almost never case for all callers?  Would this better fit in
> > > vma_modify()?  Although that's not just for this caller at this point.
> > > Maybe there isn't a good place?
> > I definitely agree with Liam that this should not be in vma_merge(), as
> > it's not going to be relevant to _most_ callers.
> >
> > I am not sure vma_modify() is much better, this would be the only early
> > exit check in that function and makes what is very simple and
> > straightforward now more confusing.
>
>
> There are two paths that will cause this case. One is in mprotect_fixup(),
> the other is in
>
> madvise_update_vma().
>
>
> One way is to separate out the split_vma() from vma_modify(). And create a
> new helper function.

Absolutely not. I wrote the vma_modify() patch series explicitly to expose
_less_ not more.

>
> We can call it directly at source, but we need to do this in both paths.
> It's more complicated.
>
>
> The other way is to add a check in vma_modify(). Like the following:

As I said above, I really don't think this is a good idea, you're just
special casing one non-merge case but not any others + adding an
unnecessary branch.

>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 0fccd23f056e..f93f1d3939f2 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2431,11 +2431,15 @@ struct vm_area_struct *vma_modify(struct
> vma_iterator *vmi,
>         pgoff_t pgoff = vma->vm_pgoff + ((start - vma->vm_start) >>
> PAGE_SHIFT);
>         struct vm_area_struct *merged;
>
> +       if (prev && end < prev->vm_end)
> +               goto cannot_merge;
> +
>         merged = vma_merge(vmi, prev, vma, start, end, vm_flags,
>                            pgoff, policy, uffd_ctx, anon_name);
>         if (merged)
>                 return merged;
>
> +cannot_merge:
>         if (vma->vm_start < start) {
>                 int err = split_vma(vmi, vma, start, 1);
>
>
> > And I think this is the crux of it - it's confusing that we special case
> > this one particular non-merge scenario, but no others (all of which we then
> > deem ok to be caught by the usual rules).
> >
> > I think it's simpler (and more efficient) to just keep things the way they
> > are.
> >
> > > Or are there other reasons this may happen and is better done in this
> > > function?
> > >
> > > Often, this is called the "punch a hole" scenario; where an operation
> > > creates two entries from the old data and either leaves an empty space
> > > or fills the space with a new VMA.
> > >
> > > > Signed-off-by: Yajun Deng <yajun.deng@xxxxxxxxx>
> > > > ---
> > > > v2: remove the case label.
> > > > v1: https://lore.kernel.org/all/20240218085028.3294332-1-yajun.deng@xxxxxxxxx/
> > > > ---
> > > >   mm/mmap.c | 3 +++
> > > >   1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > > index 0fccd23f056e..7668854d2246 100644
> > > > --- a/mm/mmap.c
> > > > +++ b/mm/mmap.c
> > > > @@ -890,6 +890,9 @@ static struct vm_area_struct
> > > >   	if (vm_flags & VM_SPECIAL)
> > > >   		return NULL;
> > > >
> > > > +	if (prev && end < prev->vm_end)
> > > > +		return NULL;
> > > > +
> > > >   	/* Does the input range span an existing VMA? (cases 5 - 8) */
> > > >   	curr = find_vma_intersection(mm, prev ? prev->vm_end : 0, end);
> > > >
> > > > --
> > > > 2.25.1
> > > >
> > So overall I don't think this check makes much sense anywhere.
> >
> > I think a better solution would be to prevent it happening _at source_ if
> > you can find a logical way of doing so.
> >
> > I do plan to do some cleanup passes over this stuff once again so maybe I
> > can figure something out that better handles non-merge scenarios.
> >
> > This is a great find though overall even if a patch doesn't make sense
> > Yajun, thanks for this, it's really made me think about this case (+ others
> > like it) :)

I guess maybe again I've not been clear enough on this, so unless
compelling reasoning can otherwise be provided, I feel this check should
not be added _anywhere_.

Therefore, NACK.