From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx121.postini.com [74.125.245.121]) by kanga.kvack.org (Postfix) with SMTP id 2F8666B004D for ; Wed, 2 May 2012 15:15:00 -0400 (EDT) Received: by ghrr18 with SMTP id r18so1481349ghr.14 for ; Wed, 02 May 2012 12:14:59 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20120502091837.GC16976@quack.suse.cz> References: <1335778207-6511-1-git-send-email-jack@suse.cz> <20120502081705.GB16976@quack.suse.cz> <20120502091837.GC16976@quack.suse.cz> From: KOSAKI Motohiro Date: Wed, 2 May 2012 15:14:33 -0400 Message-ID: Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers Content-Type: text/plain; charset=ISO-8859-1 Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: Nick Piggin , Jeff Moyer , KOSAKI Motohiro , Michael Kerrisk , LKML , linux-man@vger.kernel.org, linux-mm@kvack.org, mgorman@suse.de, Andrea Arcangeli , Woodman Hello, >> I see what you mean. >> >> I'm not sure, though. For most apps it's bad practice I think. If you get into >> realm of sophisticated, performance critical IO/storage managers, it would >> not surprise me if such concurrent buffer modifications could be allowed. >> We allow exactly such a thing in our pagecache layer. Although probably >> those would be using shared mmaps for their buffer cache. >> >> I think it is safest to make a default policy of asking for IOs against private >> cow-able mappings to be quiesced before fork, so there are no surprises >> or reliance on COW details in the mm. Do you think? > Yes, I agree that (and MADV_DONTFORK) is probably the best thing to have > in documentation. Otherwise it's a bit too hairy... I neglected this issue for years because Linus asked who need this and I couldn't find real world usecase. Ah, no, not exactly correct. Fujitsu proprietary database had such usecase. But they quickly fixed it. Then I couldn't find alternative usecase. I'm not sure why you say "hairy". Do you mean you have any use case of this? Thank you. -- 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