From: Zach Brown <zab@redhat.com>
Subject: Re: [PATCH v3] ext4: Prevent race while waling extent tree
Date: Tue, 13 Nov 2012 10:51:04 -0800
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> The deadlock mentioned by Zach Brown can be fixed by simply switching
> to GFP_NOFS.

That's a start, but it doesn't address the copy_to_user().  You could
pin that memory, I suppose, but that starts to feel like more trouble
than its worth.

- z