From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx103.postini.com [74.125.245.103]) by kanga.kvack.org (Postfix) with SMTP id 64D406B004D for ; Tue, 3 Jan 2012 21:19:00 -0500 (EST) Received: by ghrr18 with SMTP id r18so10851842ghr.14 for ; Tue, 03 Jan 2012 18:18:59 -0800 (PST) Message-ID: <4F03B715.4080005@gmail.com> Date: Tue, 03 Jan 2012 21:19:01 -0500 From: KOSAKI Motohiro MIME-Version: 1.0 Subject: Re: [PATCH 2/2] sysvshm: SHM_LOCK use lru_add_drain_all_async() References: <1325403025-22688-2-git-send-email-kosaki.motohiro@gmail.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, KOSAKI Motohiro (1/3/12 8:51 PM), Hugh Dickins wrote: > On Sun, 1 Jan 2012, kosaki.motohiro@gmail.com wrote: >> From: KOSAKI Motohiro >> >> shmctl also don't need synchrounous pagevec drain. This patch replace it with >> lru_add_drain_all_async(). >> >> Signed-off-by: KOSAKI Motohiro > > Let me answer this 2/2 first since it's easier. > > I'm going to thank you for bringing this lru_add_drain_all() > to my attention, I had not noticed it; but Nak the patch itself. > > The reason being, that particular lru_add_drain_all() serves no > useful purpose, so let's delete it instead of replacing it. I believe > that it serves no purpose for SHM_LOCK and no purpose for SHM_UNLOCK. > > I'm dabbling in this area myself, since you so cogently pointed out that > I'd tried to add a cond_resched() to scan_mapping_unevictable_pages() > (which is a helper for SHM_UNLOCK here) while it's under spinlock. > > In testing my fix for that, I find that there has been no attempt to > keep the Unevictable count accurate on SysVShm: SHM_LOCK pages get > marked unevictable lazily later as memory pressure discovers them - > which perhaps mirrors the way in which SHM_LOCK makes no attempt to > instantiate pages, unlike mlock. Ugh, you are right. I'm recovering my remember gradually. Lee implemented immediate lru off logic at first and I killed it to close a race. I completely forgot. So, yes, now SHM_LOCK has no attempt to instantiate pages. I'm ashamed. > > Since nobody has complained about that in the two years since we've > had an Unevictable count in /proc/meminfo, I don't see any need to > add code (it would need more than just your change here; would need > more even than calling scan_mapping_unevictable_pages() at SHM_LOCK > time - though perhaps along with your 1/2 that could handle it) and > overhead to satisfy a need that nobody has. > > I'll delete that lru_add_drain_all() in my patch, okay? Sure thing. :) > (But in writing this, realize I still don't quite understand why > the Unevictable count takes a second or two to get back to 0 after > SHM_UNLOCK: perhaps I've more to discover.) Interesting. I'm looking at this too. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org