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Date: Fri, 7 Mar 2014 18:51:34 +0900
From: Minchan Kim <minchan@...nel.org>
To: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc: 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 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.
>
> 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.
>
> >
> > 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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