linux-kernel - Re: [PATCH] zram: check compressor name before setting it

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Date:	Mon, 25 May 2015 13:03:04 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Marcin Jabrzyk <m.jabrzyk@...sung.com>, 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

On (05/22/15 22:14), Minchan Kim wrote:
> > > >second, there is not much value in exposing zcomp internals,
> > > >especially when the result is just another line in dmesg output.
> > > 
> > > 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.
> 
> Although Marcin's description is rather misleading, I like the patch.
> Every admin doesn't watch dmesg output. Even people could change loglevel
> simply so KERN_INFO would be void in that case.

there is no -EUNSPPORTEDCOMPRESSIONALGORITHM errno that we can return
back to userspace and expect it [userspace] to magically transform it
into a meaningful error message; users must check syslog/dmesg. that's
the way it is.

# echo LZ4 > /sys/block/zram0/comp_algorithm
# -bash: echo: write error: Device or resource busy

- hm.... why?
- well, that's why:
dmesg
[  249.745335] zram: Can't change algorithm for initialized device


> Instant error propagation is more strighforward for user point of view
> rather than delaying with depending on another event.

I'd rather just add two lines of code, w/o making zcomp internals visible.

it seems that we are trying to solve a problem that does not really
exist. I think what we really need to do is to rewrite zram documentation
and to propose zramctl usage as a recommended way of managing zram devices.
zramctl does not do `typo' errors. if somebody wants to configure zram
manually, then he simply must check syslog. it's simple.

---

 drivers/block/zram/zcomp.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index a1a8b8e..d96da53 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];
 }
 
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