From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757492AbbE3EQY (ORCPT ); Sat, 30 May 2015 00:16:24 -0400 Received: from mail-pa0-f52.google.com ([209.85.220.52]:36489 "EHLO mail-pa0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750877AbbE3EQQ (ORCPT ); Sat, 30 May 2015 00:16:16 -0400 Date: Sat, 30 May 2015 13:16:38 +0900 From: Sergey Senozhatsky To: Minchan Kim Cc: Sergey Senozhatsky , Weijie Yang , "'Andrew Morton'" , ngupta@vflare.org, "'Weijie Yang'" , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] zram: clear disk io accounting when reset zram device Message-ID: <20150530041638.GA525@swordfish> References: <"000001d099be$fae6cc90$f0b465b0$@yang"@samsung.com> <20150529034141.GA1157@swordfish> <20150529145418.GG11609@blaptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150529145418.GG11609@blaptop> 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 On (05/29/15 23:54), Minchan Kim wrote: > I think the problem is caused from weired feature "reset" of zram. agree. > Until a while ago, we didn't have hot_add/del feature so we should > use custom reset function but now we have hot/add feature. > So reset is logically same feature(ie, reset = hot_remove+hot_add > but remains same device id). > hm, sounds interesting, but I think it will end up being tricky. zram_remove() will be called from device's sysfs node (now we call it from zram_control sysfs class node, makes a huge difference). sysfs locks the node until node's read/write handler returns back, so zram_remove() will be called with lock(s_active#XXX) being locked (we had a lockdep splat with these locks recently), while zram_remove()->sysfs_remove_group() will once again attempt to lock this node (the very same lock(s_active#XXX)). in other words, we cannot fully remove zram device from its sysfs attr. and I don't want to add any bool flags to zram_remove() and zram_add() indicating that this is a "partial" device remove: don't delete device's sysfs group in remove() and don't create it in add(). doing reset from zram_control is easy, for sure: lock idr mutex, do zram_remove() and zram_add() unlock idr lock. `echo ID > /sys/.../zram_control/reset` no need to modify remove()/add() -- idr will pick up just released idx, so device_id will be preserved. but it'll be hard to drop the per-device `reset` attr and to make it a zram_control attr. things would have been much simpler if all of zram users were also zramctl users. zramctl, from this point of view, lets us change zram interfaces easily -- we merely need to teach/modify zramctl, the rest is transparent. -ss