From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: [LSF/MM TOPIC] mmap_sem in ->fault and ->page_mkwrite Date: Thu, 31 Jan 2013 23:23:35 +0100 Message-ID: <20130131222335.GA13525@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org To: lsf-pc@lists.linux-foundation.org Return-path: Received: from cantor2.suse.de ([195.135.220.15]:58852 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755385Ab3AaWXq (ORCPT ); Thu, 31 Jan 2013 17:23:46 -0500 Content-Disposition: inline Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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. Honza -- Jan Kara SUSE Labs, CR