Re: [PATCH v2 1/3] mm/hugetlb: take refcount under page table lock in follow_huge_pmd()

[Andrew Morton]

On Fri,  1 Aug 2014 13:37:41 -0400 Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> wrote:

> We have a race condition between move_pages() and freeing hugepages,
> where move_pages() calls follow_page(FOLL_GET) for hugepages internally
> and tries to get its refcount without preventing concurrent freeing.
> This race crashes the kernel, so this patch fixes it by moving FOLL_GET
> code for hugepages into follow_huge_pmd() with taking the page table lock.
> 
> This patch passes the following test. And libhugetlbfs test shows no
> regression.
> 
> ...

How were these bugs discovered?  Are we missing some Reported-by's?

> --- mmotm-2014-07-22-15-58.orig/include/linux/hugetlb.h
> +++ mmotm-2014-07-22-15-58/include/linux/hugetlb.h
> @@ -101,6 +101,8 @@ struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>  				pmd_t *pmd, int write);
>  struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
>  				pud_t *pud, int write);
> +struct page *follow_huge_pmd_lock(struct vm_area_struct *vma,
> +				unsigned long address, pmd_t *pmd, int flags);
>  int pmd_huge(pmd_t pmd);
>  int pud_huge(pud_t pmd);
>  unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
>
> ...
>
> --- mmotm-2014-07-22-15-58.orig/mm/hugetlb.c
> +++ mmotm-2014-07-22-15-58/mm/hugetlb.c
> @@ -3687,6 +3687,33 @@ follow_huge_pud(struct mm_struct *mm, unsigned long address,
>  
>  #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
>  
> +struct page *follow_huge_pmd_lock(struct vm_area_struct *vma,
> +				unsigned long address, pmd_t *pmd, int flags)

Some documentation here wouldn't hurt.  Why it exists, what it does. 
And especially: any preconditions to calling it (ie: locking).

> +{
> +	struct page *page;
> +	spinlock_t *ptl;
> +
> +	if (flags & FOLL_GET)
> +		ptl = huge_pte_lock(hstate_vma(vma), vma->vm_mm, (pte_t *)pmd);
> +
> +	page = follow_huge_pmd(vma->vm_mm, address, pmd, flags & FOLL_WRITE);
> +
> +	if (flags & FOLL_GET) {
> +		/*
> +		 * Refcount on tail pages are not well-defined and
> +		 * shouldn't be taken. The caller should handle a NULL
> +		 * return when trying to follow tail pages.
> +		 */
> +		if (PageHead(page))
> +			get_page(page);
> +		else
> +			page = NULL;
> +		spin_unlock(ptl);
> +	}
> +
> +	return page;
> +}
> +
>  #ifdef CONFIG_MEMORY_FAILURE

I can't find an implementation of follow_huge_pmd() which actually uses
the fourth argument "write".  Zap?

Ditto for follow_huge_pud().

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