From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 21 Aug 2007 02:39:22 +0200 From: Nick Piggin Subject: Re: [RFC 2/9] Use NOMEMALLOC reclaim to allow reclaim if PF_MEMALLOC is set Message-ID: <20070821003922.GD8414@wotan.suse.de> References: <20070814153021.446917377@sgi.com> <20070814153501.305923060@sgi.com> <20070818071035.GA4667@ucw.cz> <1187641056.5337.32.camel@lappy> <1187644449.5337.48.camel@lappy> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1187644449.5337.48.camel@lappy> Sender: owner-linux-mm@kvack.org Return-Path: To: Peter Zijlstra Cc: Christoph Lameter , Pavel Machek , linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, dkegel@google.com, David Miller List-ID: On Mon, Aug 20, 2007 at 11:14:08PM +0200, Peter Zijlstra wrote: > On Mon, 2007-08-20 at 13:27 -0700, Christoph Lameter wrote: > > On Mon, 20 Aug 2007, Peter Zijlstra wrote: > > > > > > Plus the same issue can happen today. Writes are usually not completed > > > > during reclaim. If the writes are sufficiently deferred then you have the > > > > same issue now. > > > > > > Once we have initiated (disk) writeout we do not need more memory to > > > complete it, all we need to do is wait for the completion interrupt. > > > > We cannot reclaim the page as long as the I/O is not complete. If you > > have too many anonymous pages and the rest of memory is dirty then you can > > get into OOM scenarios even without this patch. > > As long as the reserve is large enough to completely initialize writeout > of a single page we can make progress. Once writeout is initialized the > completion interrupt is guaranteed to happen (assuming working > hardware). Although interestingly, we are not guaranteed to have enough memory to completely initialise writeout of a single page. The buffer layer doesn't require disk blocks to be allocated at page dirty-time. Allocating disk blocks can require complex filesystem operations and readin of buffer cache pages. The buffer_head structures themselves may not even be present and must be allocated :P In _practice_, this isn't such a problem because we have dirty limits, and we're almost guaranteed to have some clean pages to be reclaimed. In this same way, networked filesystems are not a problem in practice. However network swap, because there is no dirty limits on swap, can actually see the deadlock problems. -- 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