From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [RFC] memory defragmentation to satisfy high order allocations From: Trond Myklebust In-Reply-To: <20041004.050320.78713249.taka@valinux.co.jp> References: <20041003140723.GD4635@logos.cnet> <20041004.033559.71092746.taka@valinux.co.jp> <1096831287.9667.61.camel@lade.trondhjem.org> <20041004.050320.78713249.taka@valinux.co.jp> Content-Type: text/plain; charset=iso-8859-1 Message-Id: <1096836249.9667.100.camel@lade.trondhjem.org> Mime-Version: 1.0 Date: Sun, 03 Oct 2004 22:44:09 +0200 Content-Transfer-Encoding: 8BIT Sender: owner-linux-mm@kvack.org Return-Path: To: Hirokazu Takahashi Cc: Marcelo Tosatti , iwamoto@valinux.co.jp, haveblue@us.ibm.com, Andrew Morton , linux-mm@kvack.org, piggin@cyberone.com.au, arjanv@redhat.com, linux-kernel@vger.kernel.org List-ID: Pa su , 03/10/2004 klokka 22:03, skreiv Hirokazu Takahashi: > However, while network is down network/cluster filesystems might not > release pages forever unlike in the case of block devices, which may > timeout or returns a error in case of failure. Where is the difference? As far as the VM is concerned, it is a latency problem. The fact of whether or not it is a permanent hang, a hang with a long timeout, or just a slow device is irrelevant because the VM doesn't actually know about these devices. > Each filesystem can control what the migration code does. > If it doesn't have anything to help memory migration, it's possible > to wait for the network coming up before starting memory migration, > or give up it if the network happen to be down. That's no problem. Wrong. It *is* a problem: Filesystems aren't required to know anything about the particulars of the underlying block/network/... device timeout semantics either. Think, for instance about EXT2. Where in the current code do you see that it is required to detect that it is running on top of something like the NBD device? Where does it figure out what the latencies of this device is? AFAICS, most filesystems in linux/fs/* have no knowledge whatsoever about the underlying block/network/... devices and their timeout values. Basing your decision about whether or not you need to manage high latency situations just by inspecting the filesystem type is therefore not going to give very reliable results. Cheers, Trond -- 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: aart@kvack.org