From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx198.postini.com [74.125.245.198]) by kanga.kvack.org (Postfix) with SMTP id C90BC6B0044 for ; Tue, 6 Nov 2012 22:49:02 -0500 (EST) Date: Tue, 6 Nov 2012 19:48:59 -0800 From: Andrew Morton Subject: Re: [PATCH v4 1/6] mm: teach mm by current context info to not do I/O during memory allocation Message-Id: <20121106194859.8eec3043.akpm@linux-foundation.org> In-Reply-To: References: <1351931714-11689-1-git-send-email-ming.lei@canonical.com> <1351931714-11689-2-git-send-email-ming.lei@canonical.com> <20121106152354.90150a3b.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Ming Lei Cc: linux-kernel@vger.kernel.org, Alan Stern , Oliver Neukum , Minchan Kim , Greg Kroah-Hartman , "Rafael J. Wysocki" , Jens Axboe , "David S. Miller" , netdev@vger.kernel.org, linux-usb@vger.kernel.org, linux-pm@vger.kernel.org, linux-mm@kvack.org, Jiri Kosina , Mel Gorman , KAMEZAWA Hiroyuki , Michal Hocko , Ingo Molnar , Peter Zijlstra On Wed, 7 Nov 2012 11:11:24 +0800 Ming Lei wrote: > On Wed, Nov 7, 2012 at 7:23 AM, Andrew Morton wrote: > > > > It's unclear from the description why we're also clearing __GFP_FS in > > this situation. > > > > If we can avoid doing this then there will be a very small gain: there > > are some situations in which a filesystem can clean pagecache without > > performing I/O. > > Firstly, the patch follows the policy in the system suspend/resume situation, > in which the __GFP_FS is cleared, and basically the problem is very similar > with that in system PM path. I suspect that code is wrong. Or at least, suboptimal. > Secondly, inside shrink_page_list(), pageout() may be triggered on dirty anon > page if __GFP_FS is set. pageout() should be called if GFP_FS is set or if GFP_IO is set and the IO is against swap. And that's what we want to happen: we want to enter the fs to try to turn dirty pagecache into clean pagecache without doing IO. If we in fact enter the device drivers when GFP_IO was not set then that's a bug which we should fix. > IMO, if performing I/O can be completely avoided when __GFP_FS is set, the > flag can be kept, otherwise it is better to clear it in the situation. yup. > > > > Also, you can probably put the unlikely() inside memalloc_noio() and > > avoid repeating it at all the callsites. > > > > And it might be neater to do: > > > > /* > > * Nice comment goes here > > */ > > static inline gfp_t memalloc_noio_flags(gfp_t flags) > > { > > if (unlikely(current->flags & PF_MEMALLOC_NOIO)) > > flags &= ~GFP_IOFS; > > return flags; > > } > > But without the check in callsites, some local variables will be write > two times, > so it is better to not do it. I don't see why - we just modify the incoming gfp_t at the start of the function, then use it. It gets a bit tricky with those struct initialisations. Things like struct foo bar { .a = a1, .b = b1, }; should not be turned into struct foo bar { .a = a1, }; bar.b = b1; and we don't want to do struct foo bar { }; bar.a = a1; bar.b = b1; either, because these are indeed a double-write. But we can do struct foo bar { .flags = (flags = memalloc_noio_flags(flags)), .b = b1, }; which is a bit arcane but not toooo bad. Have a think about it... -- 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