traversing vma on nommu

[Hajime Tazaki]

On Fri, 08 Nov 2024 09:39:05 +0900,
Hajime Tazaki wrote:

> > > after adding mt_validate() across various places, I found
> > > vma_iter_clear() in nommu.c triggers this issue on nommu UML.
> > > 
> > > I'm not totally understanding but if I changed the part with
> > > vma_iter_clear_gfp(), the issue (and validation error reports) are
> > > disappeared.
> > > 
> > > diff --git a/mm/nommu.c b/mm/nommu.c
> > > index 385b0c15add8..b5c11bbd69de 100644
> > > --- a/mm/nommu.c
> > > +++ b/mm/nommu.c
> > > @@ -581,7 +581,8 @@ static int delete_vma_from_mm(struct vm_area_struct *vma)
> > >  	cleanup_vma_from_mm(vma);
> > >  
> > >  	/* remove from the MM's tree and list */
> > > -	vma_iter_clear(&vmi);
> > > +	vma_iter_clear_gfp(&vmi, vma->vm_start, vma->vm_end, GFP_KERNEL);
> > > +	mt_validate(&current->mm->mm_mt);
> > >  	return 0;
> > >  }
> > > 
> > > do you think this is an appropriate fix ?
> > 
> > No.  Something is going wrong in regards to the vma iterator setup.  If
> > the values are incorrect on the preallocation then you may not have
> > enough memory to do the store.  You may end up in a situation where the
> > vma remains in the tree but the vm_file is removed from the interval
> > tree and that doesn't seem like a good idea.

sorry for asking many times.

after another random attempt trying to avoid the issue, the patch
below also fixed it.  sequential nulls are also gone.

diff --git a/mm/nommu.c b/mm/nommu.c
index 385b0c15add8..0c708f85408d 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -573,7 +573,7 @@ static int delete_vma_from_mm(struct vm_area_struct *vma)
        VMA_ITERATOR(vmi, vma->vm_mm, vma->vm_start);
 
        vma_iter_config(&vmi, vma->vm_start, vma->vm_end);
-       if (vma_iter_prealloc(&vmi, vma)) {
+       if (vma_iter_prealloc(&vmi, NULL)) {
                pr_warn("Allocation of vma tree for process %d failed\n",
                       current->pid);
                return -ENOMEM;

if this is a right fix, the following commit introduced this issue
while restructuring the interface.

commit b5df09226450165c434084d346fcb6d4858b0d52
Author: Liam R. Howlett <Liam.Howlett at oracle.com>
Date:   Mon Jul 24 14:31:52 2023 -0400

    mm: set up vma iterator for vma_iter_prealloc() calls

again, please let me know how you think.
thanks,

-- Hajime