Re: [PATCH v3] hugetlb: simplify hugetlb handling in follow_page_mask

[Mike Kravetz <mike.kravetz@oracle.com>]

On 10/26/22 17:59, Peter Xu wrote:
> Hi, Mike,
> 
> On Sun, Sep 18, 2022 at 07:13:48PM -0700, Mike Kravetz wrote:
> > +struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> > +				unsigned long address, unsigned int flags)
> > +{
> > +	struct hstate *h = hstate_vma(vma);
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	unsigned long haddr = address & huge_page_mask(h);
> > +	struct page *page = NULL;
> > +	spinlock_t *ptl;
> > +	pte_t *pte, entry;
> > +
> > +	/*
> > +	 * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> > +	 * follow_hugetlb_page().
> > +	 */
> > +	if (WARN_ON_ONCE(flags & FOLL_PIN))
> > +		return NULL;
> > +
> > +retry:
> > +	/*
> > +	 * vma lock prevents racing with another thread doing a pmd unshare.
> > +	 * This keeps pte as returned by huge_pte_offset valid.
> > +	 */
> > +	hugetlb_vma_lock_read(vma);
> 
> I'm not sure whether it's okay to take a rwsem here, as the code can be
> called by e.g. FOLL_NOWAIT?

I think you are right.  This is possible even thought not called this
way today,

> I'm wondering whether it's fine to just drop this anyway, just always walk
> it lockless.  IIUC gup callers should be safe here because the worst case
> is the caller will fetch a wrong page, but then it should be invalidated
> very soon with mmu notifiers.  One thing worth mention is that pmd unshare
> should never free a pgtable page.

You are correct in that pmd unshare will not directly free a pgtable page.
However, I think a 'very worst case' race could be caused by two threads(1,2)
in the same process A, and another process B.  Processes A and B share a PMD.
- Process A thread 1 gets a *ptep via huge_pte_offset and gets scheduled out.
- Process A thread 2 calls mprotect to change protection and unshares
  the PMD shared with process B.
- Process B then unmaps the PMD shared with process A and the PMD page
  gets deleted.
- The *ptep in Process A thread 1 then points into a freed page.
This is VERY unlikely, but I do think it is possible and is the reason I
may be overcautious about protecting against races with pmd unshare.

-- 
Mike Kravetz

> 
> IIUC it's also the same as fast-gup - afaiu we don't take the read vma lock
> in fast-gup too but I also think it's safe.  But I hope I didn't miss
> something.
> 
> -- 
> Peter Xu
>