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Re: [PATCH 3/8] xfs: initialise xfssync work before running quotachecks

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 3/8] xfs: initialise xfssync work before running quotachecks
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Mon, 26 Mar 2012 10:10:50 -0500
Cc: Ben Myers <bpm@xxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20120325232253.GJ5091@dastard>
References: <1332393313-1955-1-git-send-email-david@xxxxxxxxxxxxx> <1332393313-1955-4-git-send-email-david@xxxxxxxxxxxxx> <20120322151548.GS7762@xxxxxxx> <20120322210723.GC5091@dastard> <4F6C7BE7.3060100@xxxxxxx> <20120325232253.GJ5091@dastard>
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On 03/25/12 18:22, Dave Chinner wrote:
On Fri, Mar 23, 2012 at 08:34:31AM -0500, Mark Tinguely wrote:
>  On 03/22/12 16:07, Dave Chinner wrote:
>  >On Thu, Mar 22, 2012 at 10:15:48AM -0500, Ben Myers wrote:
>  >>On Thu, Mar 22, 2012 at 04:15:08PM +1100, Dave Chinner wrote:
>  >>>From: Dave Chinner<dchinner@xxxxxxxxxx>
>  >>>
>  >>>Because the mount process can run a quotacheck and consume lots of
>  >>>inodes, we need to be able to run periodic inode reclaim during the
>  >>>mount process. This will prevent running the system out of memory
>  >>>during quota checks.
>  >>>
>  >>>This essentially reverts 2bcf6e97, but that is safe to do now that
>  >>>the quota sync code that was causing problems during long quotacheck
>  >>>executions is now gone.
>  >>
>  >>Dave, I've held off on #s 3 and 4 because they appear to be racy.  Being
>  >
>  >What race?
>  >
>  >Cheers,
>  >
>  >Dave
>
>
>  2 of the sync workers use iterators
>    xfs_inode_ag_iterator()
>     xfs_perag_get()
>      radix_tree_lookup(&mp->m_perag_tree, agno)
>
>  The race I was worried about was in xfs_mount() to initialize the
>  mp->m_perag_lock, and the radix tree initialization:
>    INIT_RADIX_TREE(&mp->m_perag_tree, GFP_ATOMIC)).
>
>  There is a lock and 2 or 3 unbuffered I/O are performed in xfs_mountfs()
>  before the mp->m_perag_tree is initialized.
Yes they are uncached IOs so do not utilise the cache that
requires the mp->m_perag_tree to be initialised.

The point I was trying to make is the sync workers use iterators. The race is to get the mp->m_perag_tree initialized before one of the sync workers tries to do a xfs_perag_get().

I mentioned the lock and the 2 or 3 unbuffered I/O because they are the potential items that can take some time between starting the sync workers and intializing the m_perag_tree radix tree.

>  I was also looking at the xfs_perag_t being allocated in mountfs()
>  and being deallocated in umountfs(), but it turns out that is not
>  important, xfs_perag_get() will return NULL if these have not been
>  allocated yet or have been removed for the required ag.
Correct. The other side of that is that if xfssyncd is doing some
form of inode cache iteration, it will simply not find any perag
structures to scani and hence won't cause problems. Same with the
reclaim worker.

Were there any other issues?

Cheers,

Dave.
-- Dave Chinner david@xxxxxxxxxxxxx

Thanks,

--Mark Tinguely

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