Re: [PATCH 2/6] thp: optimize away unnecessary page table locking

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 27 Jan 2012 18:02:49 -0500
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> wrote:

> Currently when we check if we can handle thp as it is or we need to
> split it into regular sized pages, we hold page table lock prior to
> check whether a given pmd is mapping thp or not. Because of this,
> when it's not "huge pmd" we suffer from unnecessary lock/unlock overhead.
> To remove it, this patch introduces a optimized check function and
> replace several similar logics with it.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: David Rientjes <rientjes@google.com>
> 
> Changes since v3:
>   - Fix likely/unlikely pattern in pmd_trans_huge_stable()
>   - Change suffix from _stable to _lock
>   - Introduce __pmd_trans_huge_lock() to avoid micro-regression
>   - Return 1 when wait_split_huge_page path is taken
> 
> Changes since v2:
>   - Fix missing "return 0" in "thp under splitting" path
>   - Remove unneeded comment
>   - Change the name of check function to describe what it does
>   - Add VM_BUG_ON(mmap_sem)



> +/*
> + * Returns 1 if a given pmd maps a stable (not under splitting) thp,
> + * -1 if the pmd maps thp under splitting, 0 if the pmd does not map thp.
> + *
> + * Note that if it returns 1, this routine returns without unlocking page
> + * table locks. So callers must unlock them.
> + */


Seems nice clean up but... why you need to return (-1, 0, 1) ?

It seems the caller can't see the difference between -1 and 0.

Why not just return 0 (not locked) or 1 (thp found and locked) ?

Thanks,
-Kame

> +int __pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
> +{
> +	spin_lock(&vma->vm_mm->page_table_lock);
>  	if (likely(pmd_trans_huge(*pmd))) {
>  		if (unlikely(pmd_trans_splitting(*pmd))) {
> -			spin_unlock(&mm->page_table_lock);
> +			spin_unlock(&vma->vm_mm->page_table_lock);
>  			wait_split_huge_page(vma->anon_vma, pmd);
> +			return -1;
>  		} else {
> -			pmd_t entry;
> -
> -			entry = pmdp_get_and_clear(mm, addr, pmd);
> -			entry = pmd_modify(entry, newprot);
> -			set_pmd_at(mm, addr, pmd, entry);
> -			spin_unlock(&vma->vm_mm->page_table_lock);
> -			ret = 1;
> +			/* Thp mapped by 'pmd' is stable, so we can
> +			 * handle it as it is. */
> +			return 1;
>  		}
> -	} else
> -		spin_unlock(&vma->vm_mm->page_table_lock);
> -
> -	return ret;
> +	}
> +	spin_unlock(&vma->vm_mm->page_table_lock);
> +	return 0;
>  }
>  
>  pmd_t *page_check_address_pmd(struct page *page,
> -- 
> 1.7.7.6
> 
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