Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD

[Hillf Danton <hdanton@sina.com>]

On Wed, 11 Sep 2019 16:05:37 +0100
> 
> +#define PMD_SHARE_DISABLE_THRESHOLD	(1 << 8)
> +
>  /*
>   * Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc()
>   * and returns the corresponding pte. While this is not necessary for the
> @@ -4770,11 +4772,24 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
>  	pte_t *spte = NULL;
>  	pte_t *pte;
>  	spinlock_t *ptl;
> +	static atomic_t timeout_cnt;
>  
> -	if (!vma_shareable(vma, addr))
> -		return (pte_t *)pmd_alloc(mm, pud, addr);
> +	/*
> +	 * Don't share if it is not sharable or locking attempt timed out
> +	 * after 10ms. After 256 timeouts, PMD sharing will be permanently
> +	 * disabled as it is just too slow.
> +	 */
> +	if (!vma_shareable(vma, addr) ||
> +	   (atomic_read(&timeout_cnt) >= PMD_SHARE_DISABLE_THRESHOLD))
> +		goto out_no_share;
> +
> +	if (!i_mmap_timedlock_write(mapping, ms_to_ktime(10))) {
> +		if (atomic_inc_return(&timeout_cnt) ==
> +		    PMD_SHARE_DISABLE_THRESHOLD)
> +			pr_info("Hugetlbfs PMD sharing disabled because of timeouts!\n");
> +		goto out_no_share;
> +	}
	atomic_dec_if_positive(&timeout_cnt);

The logic to permanently disable pmd sharing does not make much sense
without anything like atomic_dec that would have been in their places,
with 256 timeouts put aside.

>  
> -	i_mmap_lock_write(mapping);
>  	vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
>  		if (svma == vma)
>  			continue;
> @@ -4806,6 +4821,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
>  	pte = (pte_t *)pmd_alloc(mm, pud, addr);
>  	i_mmap_unlock_write(mapping);
>  	return pte;
> +
> +out_no_share:
> +	return (pte_t *)pmd_alloc(mm, pud, addr);
>  }