linux-kernel - Re: [PATCH 0/3] mm: mempolicy: the failure processing about mpol_to

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Date:	Tue, 20 Aug 2013 15:48:24 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	Mel Gorman <mgorman@...e.de>, kosaki.motohiro@...fujitsu.com,
	riel@...hat.com, hughd@...gle.com, xemul@...allels.com,
	rientjes@...gle.com, Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] mm: mempolicy: the failure processing about mpol_to_str()

On 08/20/2013 02:47 PM, Cyrill Gorcunov wrote:
> On Tue, Aug 20, 2013 at 01:41:40PM +0800, Chen Gang wrote:
>>
>>> sure you'll have to change shmem_show_mpol statement to return int code.
>>> Won't this be more short and convenient?
>>>
>>>
>>
>> Hmm... if return -ENOSPC, in common processing, it still need continue
>> (but need let outside know about the string truncation).
>>
>> So I still suggest to give more check for it.
> 
> I still don't like adding additional code like
> 
> +	ret = mpol_to_str(buffer, sizeof(buffer), mpol);
> +	if (ret < 0)
> +               switch (ret) {
> +               case -ENOSPC:
> +                       printk(KERN_WARNING
> +                               "in %s: string is truncated in mpol_to_str().\n",
> +                               __func__);
> +               default:
> +                       printk(KERN_ERR
> +                               "in %s: call mpol_to_str() fail, errcode: %d. buffer: %p, size: %zu, pol: %p\n",
> +                               __func__, ret, buffer, sizeof(buffer), mpol);
> +                       return;
> +               }
> 
> this code is pretty neat for debugging purpose I think but in most case (if
> only I've not missed something obvious) it simply won't be the case.
>

For mpol_to_str(), it is for printing string, I suggest to fill buffer
as full as possible like another printing string functions, -ENOSPC is
not critical error, callers may can bear it, and still want to continue.

For 2 callers, I still suggest to process '-ENOSPC' and continue, it is
really not a critical error, they can continue.

For the 'default' error processing:

  I still suggest to 'printk' in shmem_show_mpol(), because when failure occurs, it has no return value to mark the failure to upper caller.
  Hmm... but for show_numa_map(), may remove the 'printk', only return the error code is OK. :-)


Thanks.

> Won't somthing like below do the same but with smaller code change?
> Note I've not even compiled it but it shows the idea.
> ---
>  fs/proc/task_mmu.c |    4 +++-
>  mm/shmem.c         |   17 +++++++++--------
>  2 files changed, 12 insertions(+), 9 deletions(-)
> 
> Index: linux-2.6.git/fs/proc/task_mmu.c
> ===================================================================
> --- linux-2.6.git.orig/fs/proc/task_mmu.c
> +++ linux-2.6.git/fs/proc/task_mmu.c
> @@ -1402,8 +1402,10 @@ static int show_numa_map(struct seq_file
>  	walk.mm = mm;
>  
>  	pol = get_vma_policy(task, vma, vma->vm_start);
> -	mpol_to_str(buffer, sizeof(buffer), pol);
> +	n = mpol_to_str(buffer, sizeof(buffer), pol);
>  	mpol_cond_put(pol);
> +	if (n < 0)
> +		return n;
>  
>  	seq_printf(m, "%08lx %s", vma->vm_start, buffer);
>  
> Index: linux-2.6.git/mm/shmem.c
> ===================================================================
> --- linux-2.6.git.orig/mm/shmem.c
> +++ linux-2.6.git/mm/shmem.c
> @@ -883,16 +883,20 @@ redirty:
>  
>  #ifdef CONFIG_NUMA
>  #ifdef CONFIG_TMPFS
> -static void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
> +static int shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
>  {
>  	char buffer[64];
> +	int ret;
>  
>  	if (!mpol || mpol->mode == MPOL_DEFAULT)
> -		return;		/* show nothing */
> +		return 0;	/* show nothing */
>  
> -	mpol_to_str(buffer, sizeof(buffer), mpol);
> +	ret = mpol_to_str(buffer, sizeof(buffer), mpol);
> +	if (ret < 0)
> +		return ret;
>  
>  	seq_printf(seq, ",mpol=%s", buffer);
> +	return 0;
>  }
>  
>  static struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
> @@ -951,9 +955,7 @@ static struct page *shmem_alloc_page(gfp
>  }
>  #else /* !CONFIG_NUMA */
>  #ifdef CONFIG_TMPFS
> -static inline void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
> -{
> -}
> +static inline int shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol) { return 0; }
>  #endif /* CONFIG_TMPFS */
>  
>  static inline struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
> @@ -2577,8 +2579,7 @@ static int shmem_show_options(struct seq
>  	if (!gid_eq(sbinfo->gid, GLOBAL_ROOT_GID))
>  		seq_printf(seq, ",gid=%u",
>  				from_kgid_munged(&init_user_ns, sbinfo->gid));
> -	shmem_show_mpol(seq, sbinfo->mpol);
> -	return 0;
> +	return shmem_show_mpol(seq, sbinfo->mpol);
>  }
>  #endif /* CONFIG_TMPFS */
>  
> 
> 


-- 
Chen Gang
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