From: Pavel Machek Subject: Re: [PATCH 3/3] Add timeout feature Date: Wed, 9 Jul 2008 22:44:13 +0200 Message-ID: <20080709204412.GG11006@ucw.cz> References: <20080630212450t-sato@mail.jp.nec.com> <20080701081026.GB16691@infradead.org> <20080707110730.GG5643@ucw.cz> <20080708231026.GP11558@disturbed> <20080708232031.GE18195@elf.ucw.cz> <20080709005254.GQ11558@disturbed> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii To: Christoph Hellwig , Takashi Sato , akpm@linux-foundation.org, viro@ZenIV.linux.org.uk, "linux-ext4@vger.kernel.org" Received: from gprs189-60.eurotel.cz ([160.218.189.60]:1556 "EHLO spitz.ucw.cz" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751982AbYGIU4P (ORCPT ); Wed, 9 Jul 2008 16:56:15 -0400 Content-Disposition: inline In-Reply-To: <20080709005254.GQ11558@disturbed> Sender: linux-ext4-owner@vger.kernel.org List-ID: Hi! > > > > > I still disagree with this whole patch. There is not reason to let > > > > > the freeze request timeout - an auto-unfreezing will only confuse the > > > > > hell out of the caller. The only reason where the current XFS freeze > > > > > call can hang and this would be theoretically useful is when the > > > > > > > > What happens when someone dirties so much data that vm swaps out > > > > whatever process that frozen the filesystem? > > > > > > a) you can't dirty a frozen filesystem - by definition a frozen > > > filesystem is a *clean filesystem* and *cannot be dirtied*. > > > > Can you stop me? > > > > mmap("/some/huge_file", MAP_SHARED); > > > > then write to memory mapping? > > Sure - we can put a hook in ->page_mkwrite() to prevent it. We > don't right now because nobody in the real world really cares if one > half of a concurrent user data change is in the old snapshot or the > new one...... > > > > b) Swap doesn't write through the filesystem > > > c) you can still read from a frozen filesystem to page your > > > executable?? in. > > > > atime modification should mean dirty data, right? > > Metadata, not data. If that's really a problem (and it never has > been for XFS because we always allow in memory changes to atime) > then touch_atime could be easily changed to avoid this... > > > And dirty data mean > > memory pressure, right? > > If you walk enough inodes while the filesystem is frozen, it > theoretically could happen. Typically a filesystem is only for a > few seconds at a time so in the real world this has never, ever been > a problem. So we have freezing interface that does not really freeze, and that can break the system when filesystem is frozen for too long... :-(. Maybe you could use process freezer -- cgroup people are adding userspace interface to that -- to solve those... but that would mean stopping everyone but thread doing freezing... Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html