Re: [PATCH v6 11/16] mm: enforce vma to be in detached state before freeing

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 16, 2024 at 10:16:35PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 16, 2024 at 11:24:14AM -0800, Suren Baghdasaryan wrote:
> > exit_mmap() frees vmas without detaching them. This will become a problem
> > when we introduce vma reuse. Ensure that vmas are always detached before
> > being freed.
> > 
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  kernel/fork.c |  4 ++++
> >  mm/vma.c      | 10 ++++++++--
> >  2 files changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 283909d082cb..f1ddfc7b3b48 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -473,6 +473,10 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> >  
> >  void __vm_area_free(struct vm_area_struct *vma)
> >  {
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +	/* The vma should be detached while being destroyed. */
> > +	VM_BUG_ON_VMA(!is_vma_detached(vma), vma);
> > +#endif
> >  	vma_numab_state_free(vma);
> >  	free_anon_vma_name(vma);
> >  	kmem_cache_free(vm_area_cachep, vma);
> > diff --git a/mm/vma.c b/mm/vma.c
> > index fbd7254517d6..0436a7d21e01 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -413,9 +413,15 @@ void remove_vma(struct vm_area_struct *vma, bool unreachable)
> >  	if (vma->vm_file)
> >  		fput(vma->vm_file);
> >  	mpol_put(vma_policy(vma));
> > -	if (unreachable)
> > +	if (unreachable) {
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +		if (!is_vma_detached(vma)) {
> > +			vma_start_write(vma);
> > +			vma_mark_detached(vma);
> > +		}
> > +#endif
> >  		__vm_area_free(vma);
> 
> Again, can't you race with lockess RCU lookups?

Ah, no, removing vma requires holding mmap_lock for writing and having
the vma locked, which would ensure preceding RCU readers are complete
(per the LOCK_OFFSET waiter thing) and new RCU readers are rejected for
the vma sequence thing.