Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751375AbbEYOQO (ORCPT ); Mon, 25 May 2015 10:16:14 -0400 Received: from mail-pd0-f176.google.com ([209.85.192.176]:35981 "EHLO mail-pd0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751124AbbEYOQN (ORCPT ); Mon, 25 May 2015 10:16:13 -0400 X-Google-Original-Sender: minchan.kim@gmail.com Date: Mon, 25 May 2015 23:16:05 +0900 From: Minchan Kim To: Sergey Senozhatsky Cc: Marcin Jabrzyk , Nitin Gupta , "linux-kernel@vger.kernel.org" , linux-mm , Kyungmin Park Subject: Re: [PATCH] zram: check compressor name before setting it Message-ID: <20150525141605.GC14922@blaptop> References: <1432283515-2005-1-git-send-email-m.jabrzyk@samsung.com> <20150522085523.GA709@swordfish> <555EF30C.60108@samsung.com> <20150522124411.GA3793@swordfish> <20150522131447.GA14922@blaptop> <20150525040304.GA555@swordfish> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150525040304.GA555@swordfish> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4269 Lines: 115 Hello Sergey, On Mon, May 25, 2015 at 01:03:04PM +0900, Sergey Senozhatsky wrote: > 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 The problem is that user cannot notice the fail until he try to set disksize. It's not straightforward as interface. > > - 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 is useful for people to handle zram's inteface consistenly but it couldn't be a justification for awkard error propagation which depends another event(ie, disksize setting). > zramctl does not do `typo' errors. if somebody wants to configure zram > manually, then he simply must check syslog. it's simple. If any action fails instanly, it makes sense to check it with syslog. But the problem is echo lz5 > /sys/block/zram0/comp_algorithm didn't emit any error message but could notice it when we set disksize. It's not good. Of course, if we need a lot code churn to fix it, I will think it over seriously but we could fix it simply. Why not? Frankly speaking, I didn't use zramctl ever because I didn't know when it was merged into util-linux unforunately and my distro's util-linux seems to not include it. And so many products have used zram in my company are too old to use recent util-linux. They are reluctant to update their util-linux for just zramctl. Time goes by, I belive zramctl would be standard way to use zram but it couldn't cover 100% for the world usecases. So, my point is that we should make the interface sane without dependency of zramctl. > > --- > > 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]; > } > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/