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Re: Setting project quotas on special files

To: Jeff Liu <jeff.liu@xxxxxxxxxx>, xfs@xxxxxxxxxxx
Subject: Re: Setting project quotas on special files
From: Tiziano Müller <tiziano.mueller@xxxxxxxxxxxxxxxxx>
Date: Mon, 11 Nov 2013 08:38:09 +0100
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <52807C95.7010501@xxxxxxxxxx>
Organization: stepping stone GmbH
References: <1384069188.245087.68.camel@storm> <527F63FC.1050005@xxxxxxxxxx> <1384095977.245087.88.camel@storm> <52807C95.7010501@xxxxxxxxxx>
Reply-to: tiziano.mueller@xxxxxxxxxxxxxxxxx
Hi Jeff

Am Montag, den 11.11.2013, 14:43 +0800 schrieb Jeff Liu:
> Hi Tiziano,
> 
> On 11/10 2013 23:06 PM, Tiziano MÃller wrote:
> > 
> > Hi Jeff
> > 
> > Sorry for top-posting, but it may be better to illustrate this with an
> > isolated example:
> > 
> > I have a volume named "backup" mounted with option "pquota", then I do
> > the following inside that mountpoint:
> > 
> > localhost backup # mkdir test
> > localhost backup # ln -s /invalid/location test/symlinkA
> > localhost backup # echo 42:/var/backup/test >> /etc/projects
> > localhost backup # echo test:42 >> /etc/projid
> > localhost backup # xfs_quota -x -c 'project -s test' /var/backup
> > Setting up project test (path /var/backup/test)...
> > xfs_quota: skipping special file /var/backup/test/symlinkA
> So the symlink file is skipped.
> 
> > Processed 1 (/etc/projects and cmdline) paths for project test with 
> > recursion depth infinite (-1).
> > localhost backup # cp -al test/symlinkA test/symlinkB
> > cp: cannot create hard link "test/symlinkB" to "test/symlinkA": Invalid 
> > cross-device link
> This is expected as you attempting to create a hardlink file upon a symlink
> which is located at a different volume

No, 'cp -al' does not follow the symlink (which is invalid in this
example btw) but tries to create a hardlink for that symlink.

>  and the symlink was not inherited
> from the project id(42 in this case) at this stage(i.e, it's projid is 0 as
> a default projid, which is different to 42).

Yes.

> 
> This restriction can be got from xfs_vn_link():
> 
> /*
>  * If we are using project inheritance, we only allow hard link
>  * creation in our tree when the project IDs are the same; else
>  * the tree quota mechanism could be circumvented.
>  */
>  if (unlikely((tdp->i_d.di_flags & XFS_DIFLAG_PROJINHERIT) &&
>      (xfs_get_projid(tdp) != xfs_get_projid(sip)))) {
>       error = XFS_ERROR(EXDEV);
>       goto error_return;
>  }
> 
> > localhost backup # 
> > 
> > Creating a symlink after setting up project quotas in the same directory
> > yields a different behaviour:
> > 
> > localhost backup # ln -s /invalid/location test/symlinkC
> So a symlink file is created and it is inherited the projid in this case, 
> because
> symlink file quota space is accounted according to it's path length. IOWs, it 
> would
> be reflect on xfs_quota -xc 'report -[b|h]' if you create it with a longer 
> path name
> exceeds the inline inode space.
> 
> > localhost backup # cp -al test/symlinkC test/symlinkD
> Therefore create a hardlink from this symlink file with an inherited projid is
> desired to succeed.
> 
> > localhost backup # 
> > 
> > The following shows that a symlink alone (and not its target) can have a
> > project id assigned and that the project id must be assigned to be able
> > to create a hardlink of a symlink (no matter where it points to).
> > 
> > For each of the symlinks test/symlink{A,B} run `stat` to get the inode
> > then use xfs_db on that inode to get the attributes:
> > 
> > localhost backup #  xfs_db -r -c 'inode 4362' -c 'p' /dev/vdb1  | grep 
> > projid
> > core.projid_lo = 0
> > core.projid_hi = 0
> > localhost backup #  xfs_db -r -c 'inode 4363' -c 'p' /dev/vdb1  | grep 
> > projid
> > core.projid_lo = 42
> > core.projid_hi = 0
> Just as above mentioned...
> 
> > Unfortunately xfs_io tries to follow the symlinks so it can not be used
> > to set the project manually.
> Well, why you want to set project quota via xfs_io manually? :-P.

To set the project id for special files which did not get the project id
set during "xfs_quota -x -c 'project -s test'", which was leaving my
tree in a somewhat inconsistent state wrt the project id.

As Dave wrote me on IRC, the following works (I only tested it, credit
goes to him):

1. find the inode of all special files where the project id was not set
2. determine the project id to be set by recording their parent
directorie's project id. You will end up with a tuple (for 16 bit
project ids): (inode, projid_lo), resp. a triple (for 32 bit project
ids): (inode, projid_lo, projid_hi)
3. umount the volume
4. use 'xfs_db -x -c "inode <inode>" -c "write core.projid_lo
<projid_lo>" -c "write core.projid_hi <projid_hi>" <dev>' to set the
project id on those inodes manually
5. mount the volume but without "pquota" and umount again (this will
trigger a quotacheck on the next mount with pquota)
6. mount the volume with the "pquota" option again

As for the "why can xfs_quota not set the project id on special files":
it does it via ioctl(2), which needs a file handle, which is retrieved
via open(2), which "follows" the symlinks/device files.

>  
> > 
> > What works though is renaming the file:
> > 
> > localhost backup # mv test/symlinkA test/symlinkAtmp
> > localhost backup # mv test/symlinkAtmp test/symlinkA
> rename(2) will create a new dentry as well as an inode, hence it would inherit
> the projid.
> 
> > localhost backup # stat test/symlinkA
> >   File: âtest/symlinkAâ -> â/invalid/locationâ
> >   Size: 17          Blocks: 0          IO Block: 4096   symbolic link
> > Device: fe11h/65041d        Inode: 4364        Links: 1
> > Access: (0777/lrwxrwxrwx)  Uid: (    0/    root)   Gid: (    0/    root)
> > Access: 2013-11-10 15:49:44.870068178 +0100
> > Modify: 2013-11-10 15:49:44.870068178 +0100
> > Change: 2013-11-10 16:00:54.957571504 +0100
> >  Birth: -
> > localhost backup # xfs_db -r -c 'inode 4364' -c 'p' /dev/vdb1  | grep
> > projid
> > core.projid_lo = 42
> > core.projid_hi = 0
> > localhost backup # cp -al test/symlinkA test/symlinkB
> > localhost backup # 
> In terms of project quota anyway, hardlink file is only accounted once for 
> the owner
> of the inode.  It's welcome to report back if you observed different 
> behaviour.
> 
> Thanks,
> -Jeff
> 

best regards,
Tiziano

-- 
stepping stone GmbH
Neufeldstrasse 9
CH-3012 Bern
Telefon: +41 31 332 53 63
www.stepping-stone.ch
tiziano.mueller@xxxxxxxxxxxxxxxxx

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