Re: [PATCH v4 18/33] mm: write-lock VMAs before removing them from VMA tree

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Wed, Mar 01, 2023 at 07:43:33AM +0000, Hyeonggon Yoo wrote:
> On Mon, Feb 27, 2023 at 09:36:17AM -0800, Suren Baghdasaryan wrote:
> > Write-locking VMAs before isolating them ensures that page fault
> > handlers don't operate on isolated VMAs.
> > 
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  mm/mmap.c  | 1 +
> >  mm/nommu.c | 5 +++++
> >  2 files changed, 6 insertions(+)
> > 
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 1f42b9a52b9b..f7ed357056c4 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2255,6 +2255,7 @@ int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >  static inline int munmap_sidetree(struct vm_area_struct *vma,
> >  				   struct ma_state *mas_detach)
> >  {
> > +	vma_start_write(vma);
> >  	mas_set_range(mas_detach, vma->vm_start, vma->vm_end - 1);
> 
> I may be missing something, but have few questions:
> 
> 	1) Why does a writer need to both write-lock a VMA and mark the VMA detached
> 	   when unmapping it, isn't it enough to just only write-lock a VMA?
> 
> 	2) as VMAs that are going to be removed are already locked in vma_prepare(),
> 	   so I think this hunk could be dropped?

After sending this just realized that I did not consider simple munmap case :)
But I still think 1) and 3) are valid question.

> 
> >  	if (mas_store_gfp(mas_detach, vma, GFP_KERNEL))
> >  		return -ENOMEM;
> > diff --git a/mm/nommu.c b/mm/nommu.c
> > index 57ba243c6a37..2ab162d773e2 100644
> > --- a/mm/nommu.c
> > +++ b/mm/nommu.c
> > @@ -588,6 +588,7 @@ static int delete_vma_from_mm(struct vm_area_struct *vma)
> >  		       current->pid);
> >  		return -ENOMEM;
> >  	}
> > +	vma_start_write(vma);
> >  	cleanup_vma_from_mm(vma);
> 
> 	3) I think this hunk could be dropped as Per-VMA lock depends on MMU anyway.
> 
> Thanks,
> Hyeonggon
> 
> >  
> >  	/* remove from the MM's tree and list */
> > @@ -1519,6 +1520,10 @@ void exit_mmap(struct mm_struct *mm)
> >  	 */
> >  	mmap_write_lock(mm);
> >  	for_each_vma(vmi, vma) {
> > +		/*
> > +		 * No need to lock VMA because this is the only mm user and no
> > +		 * page fault handled can race with it.
> > +		 */
> >  		cleanup_vma_from_mm(vma);
> >  		delete_vma(mm, vma);
> >  		cond_resched();
> > -- 
> > 2.39.2.722.g9855ee24e9-goog
> > 
> > 
>