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Date:	Sun, 9 Mar 2014 19:58:51 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>
Subject: Re: [PATCH] zram: propagate error to user

Hello Minchan,

On (03/07/14 18:51), Minchan Kim wrote:
> Hello Sergey!
> 
> On Fri, Mar 07, 2014 at 12:20:45PM +0300, Sergey Senozhatsky wrote:
> > On (03/07/14 10:56), Minchan Kim wrote:
> > > When we initialized zcomp with single, we couldn't change
> > > max_comp_streams without zram reset but current interface doesn't
> > > show any error to user and even it changes max_comp_streams's value
> > > without any effect so it would make user very confusing.
> > > 
> > > This patch prevents max_comp_streams's change when zcomp was
> > > initialized as single zcomp and emit the error to user(ex, echo).
> > > 
> > > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > > ---
> > >  Documentation/blockdev/zram.txt |  9 +++++----
> > >  drivers/block/zram/zcomp.c      | 10 +++++-----
> > >  drivers/block/zram/zcomp.h      |  4 ++--
> > >  drivers/block/zram/zram_drv.c   | 15 +++++++++++----
> > >  4 files changed, 23 insertions(+), 15 deletions(-)
> > > 
> > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > index 2604ffed51db..0595c3f56ccf 100644
> > > --- a/Documentation/blockdev/zram.txt
> > > +++ b/Documentation/blockdev/zram.txt
> > > @@ -37,10 +37,11 @@ Note:
> > >  In order to enable compression backend's multi stream support max_comp_streams
> > >  must be initially set to desired concurrency level before ZRAM device
> > >  initialisation. Once the device initialised as a single stream compression
> > > -backend (max_comp_streams equals to 0) changing the value of max_comp_streams
> > > -will not take any effect, because single stream compression backend implemented
> > > -as a special case and does not support dynamic max_comp_streams. Only multi
> > > -stream backend supports dynamic max_comp_streams adjustment.
> > > +backend (max_comp_streams equals to 1), you will see error if you try to change
> > > +the value of max_comp_streams because single stream compression backend
> > > +implemented as a special case by lock overhead issue and does not support
> > > +dynamic max_comp_streams. Only multi stream backend supports dynamic
> > > +max_comp_streams adjustment.
> > >  
> > >  3) Select compression algorithm
> > >  	Using comp_algorithm device attribute one can see available and
> > > diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
> > > index 92a83df40a27..15fe6a27781b 100644
> > > --- a/drivers/block/zram/zcomp.c
> > > +++ b/drivers/block/zram/zcomp.c
> > > @@ -152,7 +152,7 @@ static void zcomp_strm_multi_release(struct zcomp *comp, struct zcomp_strm *zstr
> > >  }
> > >  
> > >  /* change max_strm limit */
> > > -static int zcomp_strm_multi_set_max_streams(struct zcomp *comp, int num_strm)
> > > +static bool zcomp_strm_multi_set_max_streams(struct zcomp *comp, int num_strm)
> > >  {
> > >  	struct zcomp_strm_multi *zs = comp->stream;
> > >  	struct zcomp_strm *zstrm;
> > > @@ -171,7 +171,7 @@ static int zcomp_strm_multi_set_max_streams(struct zcomp *comp, int num_strm)
> > >  		zs->avail_strm--;
> > >  	}
> > >  	spin_unlock(&zs->strm_lock);
> > > -	return 0;
> > > +	return true;
> > >  }
> > >  
> > >  static void zcomp_strm_multi_destroy(struct zcomp *comp)
> > > @@ -231,10 +231,10 @@ static void zcomp_strm_single_release(struct zcomp *comp,
> > >  	mutex_unlock(&zs->strm_lock);
> > >  }
> > >  
> > > -static int zcomp_strm_single_set_max_streams(struct zcomp *comp, int num_strm)
> > > +static bool zcomp_strm_single_set_max_streams(struct zcomp *comp, int num_strm)
> > >  {
> > >  	/* zcomp_strm_single support only max_comp_streams == 1 */
> > > -	return -ENOTSUPP;
> > > +	return 0;
> > >  }
> > 
> > IMHO, -ENOTSUPP for unsupported operation fits better than `false'.
> > yes, currently there are only two possible returns:
> > 	0 -- success
> > 	-ENOTSUPP - not supported operation
> > 
> > though, we can extend functions later and return additional codes, other
> > than `false' and `true'.
> 
> Thing to expose to user isn't true and false but EINVAL.
> 

sure. I mean we can return actual zcomp_set_max_streams() error (if any)
back to user from max_comp_streams_store():

	[..]
	ret = zcomp_set_max_streams(...);
	[..]
	return ret;

> > 
> > for example, -E2BIG if user requested extremly large number of streams,
> > like 5000 streams.
> 
> I'm not sure it's right example for E2BIG.
> When I read the comment, it says "argument list too long".
> Anyway, when I tried ENOTSUPP, echo doesn't show meaningful error to user
> and I dont' know it's casual err number for userspace.
> Pz, comment about that.
> 

E2BIG (introduced by POSIX.1-2001) was a quick example and, probably, not
the perfect one. In general, I'm not against this change and I can live
with zcomp_set_max_streams() returning bool.

	-ss

> > 
> > >  
> > >  static void zcomp_strm_single_destroy(struct zcomp *comp)
> > > @@ -283,7 +283,7 @@ ssize_t zcomp_available_show(const char *comp, char *buf)
> > >  	return sz;
> > >  }
> > >  
> > > -int zcomp_set_max_streams(struct zcomp *comp, int num_strm)
> > > +bool zcomp_set_max_streams(struct zcomp *comp, int num_strm)
> > >  {
> > >  	return comp->set_max_streams(comp, num_strm);
> > >  }
> > > diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
> > > index 8b8997f8613b..c59d1fca72c0 100644
> > > --- a/drivers/block/zram/zcomp.h
> > > +++ b/drivers/block/zram/zcomp.h
> > > @@ -46,7 +46,7 @@ struct zcomp {
> > >  
> > >  	struct zcomp_strm *(*strm_find)(struct zcomp *comp);
> > >  	void (*strm_release)(struct zcomp *comp, struct zcomp_strm *zstrm);
> > > -	int (*set_max_streams)(struct zcomp *comp, int num_strm);
> > > +	bool (*set_max_streams)(struct zcomp *comp, int num_strm);
> > >  	void (*destroy)(struct zcomp *comp);
> > >  };
> > >  
> > > @@ -64,5 +64,5 @@ int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
> > >  int zcomp_decompress(struct zcomp *comp, const unsigned char *src,
> > >  		size_t src_len, unsigned char *dst);
> > >  
> > > -int zcomp_set_max_streams(struct zcomp *comp, int num_strm);
> > > +bool zcomp_set_max_streams(struct zcomp *comp, int num_strm);
> > >  #endif /* _ZCOMP_H_ */
> > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > index 7631ef0cbc41..eead9db9b100 100644
> > > --- a/drivers/block/zram/zram_drv.c
> > > +++ b/drivers/block/zram/zram_drv.c
> > > @@ -128,17 +128,24 @@ static ssize_t max_comp_streams_store(struct device *dev,
> > >  	struct zram *zram = dev_to_zram(dev);
> > >  
> > >  	if (kstrtoint(buf, 0, &num))
> > > -		return -EINVAL;
> > > +		goto out;
> > >  	if (num < 1)
> > > -		return -EINVAL;
> > > +		goto out;
> > > +
> > >  	down_write(&zram->init_lock);
> > >  	if (init_done(zram)) {
> > > -		if (zcomp_set_max_streams(zram->comp, num))
> > > +		if (!zcomp_set_max_streams(zram->comp, num)) {
> > >  			pr_info("Cannot change max compression streams\n");
> > > +			goto out_unlock;
> > > +		}
> > >  	}
> > > +
> > >  	zram->max_comp_streams = num;
> > 
> > agree, better to avoid this for a single stream.
> 
> Yeb.
> 
> -- 
> Kind regards,
> Minchan Kim
> 
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