Re: [RFC PATCH] mm: Fix a huge pud insertion race during faulting

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Oct 08, 2019 at 11:37:11AM +0200, Thomas Hellström (VMware) wrote:
> From: Thomas Hellstrom <thellstrom@vmware.com>
> 
> A huge pud page can theoretically be faulted in racing with pmd_alloc()
> in __handle_mm_fault(). That will lead to pmd_alloc() returning an
> invalid pmd pointer. Fix this by adding a pud_trans_unstable() function
> similar to pmd_trans_unstable() and check whether the pud is really stable
> before using the pmd pointer.
> 
> Race:
> Thread 1:             Thread 2:                 Comment
> create_huge_pud()                               Fallback - not taken.
> 		      create_huge_pud()         Taken.
> pmd_alloc()                                     Returns an invalid pointer.
> 
> Cc: Matthew Wilcox <willy@infradead.org>
> Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages")
> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
> ---
> RFC: We include pud_devmap() as an unstable PUD flag. Is this correct?
>      Do the same for pmds?

I *think* it is correct and we should do the same for PMD, but I may be
wrong.

Dan, Matthew, could you comment on this?

> ---
>  include/asm-generic/pgtable.h | 25 +++++++++++++++++++++++++
>  mm/memory.c                   |  6 ++++++
>  2 files changed, 31 insertions(+)
> 
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index 818691846c90..70c2058230ba 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -912,6 +912,31 @@ static inline int pud_trans_huge(pud_t pud)
>  }
>  #endif
>  
> +/* See pmd_none_or_trans_huge_or_clear_bad for discussion. */
> +static inline int pud_none_or_trans_huge_or_dev_or_clear_bad(pud_t *pud)
> +{
> +	pud_t pudval = READ_ONCE(*pud);
> +
> +	if (pud_none(pudval) || pud_trans_huge(pudval) || pud_devmap(pudval))
> +		return 1;
> +	if (unlikely(pud_bad(pudval))) {
> +		pud_clear_bad(pud);
> +		return 1;
> +	}
> +	return 0;
> +}
> +
> +/* See pmd_trans_unstable for discussion. */
> +static inline int pud_trans_unstable(pud_t *pud)
> +{
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) &&			\
> +	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
> +	return pud_none_or_trans_huge_or_dev_or_clear_bad(pud);
> +#else
> +	return 0;
> +#endif
> +}
> +
>  #ifndef pmd_read_atomic
>  static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
>  {
> diff --git a/mm/memory.c b/mm/memory.c
> index b1ca51a079f2..43ff372f4f07 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3914,6 +3914,7 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  	vmf.pud = pud_alloc(mm, p4d, address);
>  	if (!vmf.pud)
>  		return VM_FAULT_OOM;
> +retry_pud:
>  	if (pud_none(*vmf.pud) && __transparent_hugepage_enabled(vma)) {
>  		ret = create_huge_pud(&vmf);
>  		if (!(ret & VM_FAULT_FALLBACK))
> @@ -3940,6 +3941,11 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  	vmf.pmd = pmd_alloc(mm, vmf.pud, address);
>  	if (!vmf.pmd)
>  		return VM_FAULT_OOM;
> +
> +	/* Huge pud page fault raced with pmd_alloc? */
> +	if (pud_trans_unstable(vmf.pud))
> +		goto retry_pud;
> +
>  	if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) {
>  		ret = create_huge_pmd(&vmf);
>  		if (!(ret & VM_FAULT_FALLBACK))
> -- 
> 2.20.1
> 

-- 
 Kirill A. Shutemov