Re: [PATCH] mm/mremap: Fix move_normal_pmd/retract_page_tables race

[David Hildenbrand]

On 08.10.24 05:53, Qi Zheng wrote:
> Hi Jann,
>
> On 2024/10/8 05:42, Jann Horn wrote:
>
> [...]
>
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 24712f8dbb6b..dda09e957a5d 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -238,6 +238,7 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
> >    {
> >    	spinlock_t *old_ptl, *new_ptl;
> >    	struct mm_struct *mm = vma->vm_mm;
> > +	bool res = false;
> >    	pmd_t pmd;
> >    
> >    	if (!arch_supports_page_table_move())
> > @@ -277,19 +278,25 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
> >    	if (new_ptl != old_ptl)
> >    		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> >    
> > -	/* Clear the pmd */
> >    	pmd = *old_pmd;
> > +
> > +	/* Racing with collapse? */
> > +	if (unlikely(!pmd_present(pmd) || pmd_leaf(pmd)))
>
> Since we already hold the exclusive mmap lock, after a racing
> with collapse occurs, the pmd entry cannot be refilled with
> new content by page fault. So maybe we only need to recheck
> pmd_none(pmd) here?

My thinking was that it is cheap and more future proof to check that we 
really still have a page table here. For example, what if collapse code 
is ever changed to replace the page table by the collapsed PMD?

So unless there is a good reason not to have this check here, I would 
keep it like that.

Thanks!

--
Cheers,

David / dhildenb