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Re: [PATCH 50/55] xfs: di_flushiter considered harmful

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 50/55] xfs: di_flushiter considered harmful
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Fri, 06 Sep 2013 13:43:35 -0500
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1378332359-14737-51-git-send-email-david@xxxxxxxxxxxxx>
References: <1378332359-14737-1-git-send-email-david@xxxxxxxxxxxxx> <1378332359-14737-51-git-send-email-david@xxxxxxxxxxxxx>
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On 09/04/13 17:05, Dave Chinner wrote:
From: Dave Chinner<dchinner@xxxxxxxxxx>

When we made all inode updates transactional, we no longer needed
the log recovery detection for inodes being newer on disk than the
transaction being replayed - it was redundant as replay of the log
would always result in the latest version of the inode woul dbe on
disk. It was redundant, but left in place because it wasn't
considered to be a problem.

However, with the new "don't read inodes on create" optimisation,
flushiter has come back to bite us. Essentially, the optimisation
made always initialises flushiter to zero in the create transaction,
and so if we then crash and run recovery and the inode already on
disk has a non-zero flushiter it will skip recovery of that inode.
As a result, log recovery does the wrong thing and we end up with a
corrupt filesystem.

Because we have to support old kernel to new kernl upgrades, we
can't just get rid of the flushiter support in log recovery as we
might be upgrading from a kernel that doesn't have fully transaction
inode updates.  Unfortunately, for v4 superblocks there is no way to
guarantee that log recovery knows about this fact.

We cannot add a new inode format flag to say it's a "special inode
create" because it won't be understood by older kernels and so
recovery could do the wrong thing on downgrade. We cannot specially
detect the combination of zero mode/non-zero flushiter on disk to
non-zero mode, zero flushiter in the log item during recovery
because wrapping of the flushiter can result in false detection.

Hence that makes this "don't use flushiter" optimisation limited to
a disk format that guarantees that we don't need it. And that means
the only fix here is to limit the "no read IO on create"
optimisation to version 5 superblocks....

Signed-off-by: Dave Chinner<dchinner@xxxxxxxxxx>

user space portion of kernel commit e60896
Looks good.

Reviewed-by: Mark Tinguely <tinguely@xxxxxxx>

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