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Re: [PATCH] Re-dirty pages on I/O error

To: Lachlan McIlroy <lachlan@xxxxxxx>
Subject: Re: [PATCH] Re-dirty pages on I/O error
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 16 Sep 2008 14:01:25 +1000
Cc: xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <48CDD4EE.8040105@xxxxxxx>
Mail-followup-to: Lachlan McIlroy <lachlan@xxxxxxx>, xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
References: <48C8D8CD.7050508@xxxxxxx> <20080913041930.GC5811@disturbed> <48CDD4EE.8040105@xxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
On Mon, Sep 15, 2008 at 01:22:22PM +1000, Lachlan McIlroy wrote:
> Dave Chinner wrote:
>> So we keep dirty pages around that we can't write back?
> Yes.
>> If we are in a low memory situation and the block device
>> has gone bad, that will prevent memory reclaim from making
>> progress.
> How do you differentiate "gone bad" from temporarily unavailable?

The only "temporary" error you can get in writeback is a path
failure. IIRC, XVM will give an ENODEV on a path failure, but
I don't think that dm-multipath does. Other than that, a write
failure is unrecoverable. Any other error is permanent....

>> i.e. if we have a bad disk, a user can now take down the system
>> by running it out of clean memory....
> I'm sure there's many ways a malicious user could already do that.

That's no excuse for introducing a new way of taking down the
system when a disk fails. Error handling in linux is bad enough
without intentionally preventing the system from recovering from
I/O errors...

> Would you rather have data corruption?

Data corruption as a result of an I/O error? What else can we
be expected to do? Log the error and continue onwards....

Face it - if the drive is dead then we can't write the data
anywhere, so keeping it around and potentially killing the system
completely makes even less sense.  At some point we *have to give
up* on data we can't write back....

> We've allowed the write() to succeed.  We've accepted the data.
> We have an obligation to write it do disk.  Either we keep trying
> in the face of errors or we take down the filesystem.

It's write-behind buffering. We give best effort, not guaranteed
writeback. If the system crashes, that data is lost. If we get an
I/O error, that data is lost. If the application cares, it uses
fsync and it gets the error and can handle it.


> The EAGAIN case can be exceptioned.  The error we are getting here
> is ENOSPC because xfs_trans_reserve() is failing. 


Please - put that detail in the patch description. I'm getting a
little tired of having to draw out the reasons for your patches
one little bit at a time.

So: why is xfs_trans_reserve() failing? Aren't all the transactions
in the writeback path marked as XFS_TRANS_RESERVE? That allows the
transaction reserve to succeed when at ENOSPC by dipping into the
reserved blocks. Did we run out of reserved blocks (i.e. the reserve
pool is not big enough)? Or is there some other case that leads to
ENOSPC in the writeback path that we've never considered before?


Dave Chinner

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