From: Surbhi Palande Subject: Re: your mail Date: Tue, 03 May 2011 16:56:57 +0300 Message-ID: <4DC009A9.20308@canonical.com> References: <4DBFE09E.5070805@canonical.com> <1304428117-6195-1-git-send-email-surbhi.palande@canonical.com> <20110503134636.GA6009@quack.suse.cz> Reply-To: surbhi.palande@canonical.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: toshi.okajima@jp.fujitsu.com, tytso@mit.edu, m.mizuma@jp.fujitsu.com, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, sandeen@redhat.com To: Jan Kara Return-path: In-Reply-To: <20110503134636.GA6009@quack.suse.cz> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 05/03/2011 04:46 PM, Jan Kara wrote: > On Tue 03-05-11 16:08:36, Surbhi Palande wrote: Sorry for missing the subject line :( >> On munmap() zap_pte_range() is called which dirties the PTE dirty pages as >> Toshiyuki pointed out. >> >> zap_pte_range() >> mapping->a_ops->set_page_dirty (= ext4_journalled_set_page_dirty) >> >> So, I think that it is here that we should do the checking for a ext4 F.S >> frozen state and also prevent a parallel ext4 F.S freeze from happening. >> >> Attaching a patch for initial review. Please do let me know your thoughts! > This is definitely the wrong place. ->set_page_dirty() callbacks are > called with various locks held and the page need not be locked (thus > dereferencing page->mapping is oopsable). Moreover this particular callback > is called only in data=journal mode. Ok! Thanks for that! > > Believe me, the right place is page_mkwrite() - you have to catch the > read-only => read-write page transition. Once the page is mapped > read-write, you've already lost the race. My only point is: 1) something should prevent the freeze from happening. We cant merely check the vfs_check_frozen()? And this should be done where the page is marked dirty.Also, I thought that the page is marked read-write only in the page table in the __do_page_fault()? i.e the zap_pte_range() marks them dirty in the page cache? Is this understanding right? IMHO, whatever code dirties the page in the page cache should call a F.S specific function and let it _prevent_ a fsfreeze while the page is getting dirtied, so that a freeze called after this point flushes this page! Warm Regards, Surbhi. > > Honza