From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx144.postini.com [74.125.245.144]) by kanga.kvack.org (Postfix) with SMTP id 7A2656B0005 for ; Thu, 31 Jan 2013 18:03:33 -0500 (EST) Date: Thu, 31 Jan 2013 23:03:27 +0000 From: Al Viro Subject: Re: [LSF/MM TOPIC] mmap_sem in ->fault and ->page_mkwrite Message-ID: <20130131230327.GN4503@ZenIV.linux.org.uk> References: <20130131222335.GA13525@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130131222335.GA13525@quack.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org On Thu, Jan 31, 2013 at 11:23:35PM +0100, Jan Kara wrote: > Hi, > > I'm not sure if this is such a great topic but it's a question which > I came across a few times already and LSF/MM is a good place for > brainstorming somewhat crazy ideas ;). > > So currently ->fault() and ->page_mkwrite() are called under mmap_sem held > for reading. Now this creates sometimes unpleasant locking dependencies for > filesystems (modern filesystems have to do an equivalent of ->write_begin > in ->page_mkwrite and that is a non-trivial operation). Just to mention my > last itch, I had to split reader side of filesystem freezing lock into two > locks - one which ranks above mmap_sem and one which ranks below it. Then > writer side has to wait for both locks. It works but ... > > So I was wondering: Would it be somehow possible we could drop mmap_sem in > these two callbacks (especially ->page_mkwrite())? I understand process' > mapping can change under us once we drop the semaphore so we'd have to > somehow recheck we have still the right page after re-taking mmap_sem. Like > if we protected VMAs with SRCU so that they don't disappear under us once > we drop mmap_sem and after retaking mmap_sem we would recheck whether VMA > still applies to our fault. > > And I know there's VM_FAULT_RETRY but that really seems like a special hack > for x86 architecture page fault code. Making it work for all architectures > and callers such as get_user_pages() didn't really seem plausible to me. Please, *please*, don't. VMA locking is complete horror without SRCU mess thrown in. It's a bloody bad idea, at least without a very massive cleanup prior to that thing. Start with drawing the call graph for vma-related code - at least the parts from relevant locks grabbed to accesses of fields protected by said locks. -- 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