linux-ext4 - Re: Journal under-reservation bug on first >2G file

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Message-ID: <542B1EFC.4050500@redhat.com>
Date:	Tue, 30 Sep 2014 16:22:04 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Journal under-reservation bug on first >2G file

On 9/30/14 4:10 PM, Eric Sandeen wrote:
> Hey all -
> 
> So the following testcase will overrun the 1-credit journal reservation
> made during a delalloc write in ext4_da_write_begin(), because we
> may cross the 2G threshold, and need to modify both the inode and the
> superblock in the same transaction.
> 
> I see a few was to fix this:
> 
> 1) Always set LARGE_FILE on mount if not set.  This will break
>    RW compatiblity with very old kernels.  Do we care?

  1.5) Don't update the feature on the fly - we don't for
       HUGE_FILE, either.

  1.5a) Always set the large_file feature with a fresh mkfs, insteadl
        of relying on the accident of the resize inode being > 2G!

> 2) Bump the reservation to 2 under the fiddly condition of
>    large file not yet set but this write might do it
> 3) bump the delalloc reservation to 2 just in case, always
> 
> I'll be happy to write the patch to fix it, just wondering what
> people think the best approach is
> 
> Thoughts?
> -Eric
> 
> 
> #!/bin/bash
> 
> # A 400m fs won't get the large_file feature, oddly
> # enough, because the resize inode will be < 2G.
> 
> truncate --size=400m test.img
> mkfs.ext4 -F test.img
> # This shouldn't have large_file set, exit if it does for some reason
> dumpe2fs -h test.img | grep large_file && exit
> 
> mkdir -p mnt
> mount -o loop test.img mnt
> 
> echo "writing 1 byte at 2147483646" 
> dd if=/dev/zero of=mnt/testfile bs=1 seek=2147483646 count=1 conv=notrunc of=mnt/testfile
> sync
> 
> # This will make sure i_disksize is on disk, and
> # that the buffer will be mapped on the next write.
> #
> # This is critical because ext4_da_should_update_i_disksize()
> # checks buffer_mapped():
> #
> #        if (!buffer_mapped(bh) || (buffer_delay(bh)) || buffer_unwritten(bh))
> #                return 0;
> #        return 1;
> 
> # This tries to update i_disksize, and also requires a superblock
> # update for the large_file feature flag, but only has 1 credit
> # available on the delalloc write path
> 
> echo "writing 1 byte at 2147483647"
> dd if=/dev/zero of=mnt/testfile bs=1 seek=2147483647 count=1 conv=notrunc of=mnt/testfile
> 
> # Should go boom, but if not, unmount
> umount mnt
> --
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