Date:	Mon, 25 May 2015 09:15:00 +0200
From:	Marcin Jabrzyk <m.jabrzyk@...sung.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	minchan@...nel.org, ngupta@...are.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kyungmin.park@...sung.com
Subject: Re: [PATCH] zram: check compressor name before setting it

Hi Sergey,

On 25/05/15 08:18, Sergey Senozhatsky wrote:
> On (05/22/15 15:26), Marcin Jabrzyk wrote:
>>>>  From the other hand, the only valid values that can be written are
>>>> in 'comp_algorithm'.
>>>> So when writing other one, returning -EINVAL seems to be reasonable.
>>>> The user would get immediately information that he can't do that,
>>>> now the information can be very deferred in time.
>>>
>>> it's not.
>>> the error message appears in syslog right before we return -EINVAL
>>> back to user.
>>
>> Yes I've read up the code more detailed and I saw that error message
>> just before returning to user with error value.
>>
>> But this happens when 'disksize' is wirtten, not when 'comp_algorithm'.
>> I understood, the error message in dmesg is clear there is no such
>> algorithm.
>>
>> But this is not an immediate error, when setting the 'comp_algorithm',
>> where we already know that it's wrong, not existing etc.
>> Anything after that moment would be wrong and would not work at all.
>>
>>  From what I saw 'comp_algorithm_store' is the only *_store in zram that
>> believes user that he writes proper value and just makes strlcpy.
>>
>> So what I've ing mind is to provide direct feedback, you have
>> written wrong name of compressor, you got -EINVAL, please write
>> correct value. This would be very useful when scripting.
>>
>
> I can't see how printing error 0.0012 seconds earlier helps. really.
> if one sets a compression algorithm the very next thing to do is to
> set device's disksize. even if he/she usually watch a baseball game in
> between, then the error message appears right when it's needed anyway:
> during `setup my device and make it usable' stage.
>
>
>> I'm not for exposing more internals, but getting -EINVAL would be nice I
>
> you are.
>
> find_backend() returns back to its caller a raw and completely initialized
> zcomp_backend pointer. this is very dangerous zcomp internals, which should
> never be exposed. from zcomp layer we return either ERR_PTR() or a usable
> zcomp_backend pointer. that's the rule.
>
>
> if you guys still insist that this is critical and very important change,
> then there should be a small helper function instead with a clear name
> (starting with zcomp_ to indicate its place) which will simply return bool
> TRUE for `algorithm is known' case and FALSE otherwise (aka `algorithm is
> unknown').
>
This is exactly the idea I've in my mind, when thinking about much 
proper solution.
Simple function that will return bool and just check if name is correct 
or not. Without presenting find_backend or anything from zcomp.

>
> something like below:
>
>
>    # echo LZ5 > /sys/block/zram0/comp_algorithm
>    -bash: echo: write error: Invalid argument
>
>    dmesg
>    [ 7440.544852] Error: unknown compression algorithm: LZ5
>
>
> p.s. but, I still see a very little value.
> p.p.s notice a necessary and inevitable `dmesg'. (back to 'no one reads
> syslog' argument).
>
> ---
>
>   drivers/block/zram/zcomp.c    | 10 ++++++++++
>   drivers/block/zram/zcomp.h    |  1 +
>   drivers/block/zram/zram_drv.c |  3 +++
>   3 files changed, 14 insertions(+)
>
> diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
> index a1a8b8e..b68b16f 100644
> --- a/drivers/block/zram/zcomp.c
> +++ b/drivers/block/zram/zcomp.c
> @@ -54,11 +54,16 @@ static struct zcomp_backend *backends[] = {
>   static struct zcomp_backend *find_backend(const char *compress)
>   {
>   	int i = 0;
> +
>   	while (backends[i]) {
>   		if (sysfs_streq(compress, backends[i]->name))
>   			break;
>   		i++;
>   	}
> +
> +	if (!backends[i])
> +		pr_err("Error: unknown compression algorithm: %s\n",
> +				compress);
>   	return backends[i];
>   }
>
> @@ -320,6 +325,11 @@ void zcomp_destroy(struct zcomp *comp)
>   	kfree(comp);
>   }
>
> +bool zcomp_known_algorithm(const char *comp)
> +{
> +	return find_backend(comp) != NULL;
> +}
> +
>   /*
>    * search available compressors for requested algorithm.
>    * allocate new zcomp and initialize it. return compressing
> diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
> index c59d1fc..773bdf1 100644
> --- a/drivers/block/zram/zcomp.h
> +++ b/drivers/block/zram/zcomp.h
> @@ -51,6 +51,7 @@ struct zcomp {
>   };
>
>   ssize_t zcomp_available_show(const char *comp, char *buf);
> +bool zcomp_known_algorithm(const char *comp);
>
>   struct zcomp *zcomp_create(const char *comp, int max_strm);
>   void zcomp_destroy(struct zcomp *comp);
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index f750e34..2197a81 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -378,6 +378,9 @@ static ssize_t comp_algorithm_store(struct device *dev,
>   	if (sz > 0 && zram->compressor[sz - 1] == '\n')
>   		zram->compressor[sz - 1] = 0x00;
>
> +	if (!zcomp_known_algorithm(zram->compressor))
> +		len = -EINVAL;
> +
>   	up_write(&zram->init_lock);
>   	return len;
>   }
>
I'm perfectly fine with this solution. It just does what
I'd expect.

Best regards,
Marcin Jabrzyk

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