From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dongsheng Yang Subject: Re: [PATCH v3 11/39] fs: quota: replace opened calling of ->sync_fs with sync_filesystem Date: Mon, 21 Sep 2015 12:31:39 +0800 Message-ID: <55FF882B.4030003@cn.fujitsu.com> References: <1442307754-13233-1-git-send-email-yangds.fnst@cn.fujitsu.com> <1442307754-13233-12-git-send-email-yangds.fnst@cn.fujitsu.com> <20150916101402.GE13325@quack.suse.cz> <55FA5D8A.9030108@cn.fujitsu.com> <20150917110547.GB32280@quack.suse.cz> <55FBA5E5.1040201@cn.fujitsu.com> <20150918090038.GA14330@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Cc: , , , , To: Jan Kara Return-path: Received: from cn.fujitsu.com ([59.151.112.132]:61281 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1756006AbbIUEiF (ORCPT ); Mon, 21 Sep 2015 00:38:05 -0400 In-Reply-To: <20150918090038.GA14330@quack.suse.cz> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 09/18/2015 05:00 PM, Jan Kara wrote: > On Fri 18-09-15 13:49:25, Dongsheng Yang wrote: >> On 09/17/2015 07:05 PM, Jan Kara wrote: >>> On Thu 17-09-15 14:28:26, Dongsheng Yang wrote: >>>> On 09/16/2015 06:14 PM, Jan Kara wrote: >>>>> On Tue 15-09-15 17:02:06, Dongsheng Yang wrote: >>>>>> There are only two places in whole kernel to call ->sync_fs directly. It >>>>>> will sync fs even in read-only mode. It's not a good idea and some filesystem >>>>>> would warn out if you are syncing in read-only mode. But sync_filesystem() >>>>>> does filter this case out. Let's call sync_filesystem() here instead. >>>>>> >>>>>> Signed-off-by: Dongsheng Yang >>>>> >>>>> So I'd prefer ubifs used hidden system inodes for quota files, set >>>>> DQUOT_QUOTA_SYS_FILE flag and so this code calling sync_fs() could be >>>>> completely avoided. >>>> >>>> Hmmm, I think it's a good idea to make quota files SYS_FILEs. But how >>>> about quota-tools? E.g, if quotacheck do some modification >>>> on quota files, we would not read them without a sync_fs(). >>>> >>>> Could you help explain how quota works in this case? >>> >>> So tools like quota(1) or setquota(1) work using quotactl so they don't >>> need access to quota files. When quota files are system files, quotacheck >>> functionality belongs into the fsck - so fsck.ubifs is responsible for >>> checking consistency of quota files. How e.g. e2fsck does it is that when >>> scanning inodes, it computes usage for each user / group, loads limits >>> information from old quota files and then just creates new quota files with >>> updated information if there's any difference to the old quota files. >> >> About quotacheck, I think just call fsync() in it sounds good to me. >> Maybe something like the attachment. >> >> OKEY, but I found repquota is still using read() to access quota files. > > repquota(8) uses read on quota files only if quota files are visible to > userspace. If quota files are invisible (they are system files), > repquota(8) uses Q_GETQUOTA quotactl to report quotas. So this is not an > issue. > >> Please consider that case: 1.we read quota file and there is a pagecache >> for it. 2. Then kernel did some modification on quota files. But >> if the files is SYS_FILES marked, dquot_quota_sync() would not drop >> the pagecache, then 3. repquota would get an outdated data from pagecache. > > You can mark quota files as SYS_FILES only if they are invisible to > userspace - usually they are stored in inodes not attached to any directory > (their inode numbers are stored in superblock or similarly so that fs can > find them when it is mounted). > >> I am not sure why ext4 works well in this case. There must be something >> I am missing. > > Ext4 marks quota files as SYS_FILES only if it is configured to have quota > files in hidden system inodes. So everything works fine. > >> Maybe we can introduce a Q_GETBLK for quotactl() to make all >> quota tools getting informations from ioctl. > > The idea is we don't want to expose raw quota blocks to quota tools > anymore and there's no real need to. The only suboptimal thing is > repquota(8) which would benefit from a way to iterate over all quota > entries of a particular quota type to make it more efficient if it doesn't > have access to quota file. For that we could create a new quotactl > but it was never pressing enough so that it would get implemented. > >>> Another advantage of the checking functionality being in fsck is that >>> fs-specific fsck can gather usage information much more efficiently (and >>> fsck has to do full fs scan anyway) and there's no need to propagate quota >>> usage information to userspace using FIQSIZE ioctl() and similar stuff... >> >> So, let me try to summary the my opinions about it. >> Pros: >> (1). Security. quota files shouldn't be accessible to usespace. >> (2). Efficiency. No need for quotacheck, just do it in fsck. >> (3). No need FIOQSIZE any more. >> Cons: >> (1). Incompatibility: >> If I set DQUOT_QUOTA_SYS_FILE currently, there are at >> least two commands would not work: quotacheck and repquota. Actually >> that means the whole quota is not usable to user. > > When quota file is a system file, quotacheck(8) is not needed - fsck does > all the work. So that isn't an issue. Repquota(8) knows how to deal with a > situation when quota file is not visible (when it is a system file), so > that works as well. Believe me, both ext4 and ocfs2 use this mechanism and > it works fine. XFS and GFS2 use the same principles but the don't fully use > the quota framework (only quotactl interface) so that's a bit different > case. > >> So, I think the compatibility is important to me. Then what about >> setting quota files as regular files at first. After all tools (quota >> tools, quotacheck, repquota, fsck) prepared, setting the >> DQUOT_QUOTA_SYS_FILE seems better. > > Quota tools are prepared for this. And ext4 originally used quota files as > regular files and later transitioned to store quota in system files and the > transition isn't a trivial process so it's better to avoid it. Okey, so I need to do my work similar with what ext4 is doing. I will read more about it in ext4 and quota-tools to see how to implement it in ubifs. > > Furthermore I'm not inclined to accept changes to generic quota > infrastructure just to accommodate ubifs to store quotas in regular files > when storing them in system files will solve these problems without changes > to the generic infrastructure. Yes, I agree with that. We should keep the generic infrastructure until we have to change it. :) Yang > > Honza >