Re: bug#8411: due to missing sync even on 2.6.39, cp fails to copy an odd file

Jim Meyering
2011-04-02T13:50:22Z

Pádraig Brady wrote:
> On 02/04/11 12:16, Jim Meyering wrote:
>> Hi Pádraig,
>> As of this change,
>>
>> copy: with fiemap copy, only sync when needed
>> http://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=f69876e55
>>
>> fiemap copy with extents beyond EOF can fail on ext4 even with
>> Fedora 15 (2.6.38) and rawhide's 2.6.39 kernel.
>>
>> Here we construct an odd file. First, preallocate 10MB of space,
>> and then write 5KiB of random data into the beginning of that:
>>
>> $ fallocate -l 10000000 -n k
>> $ dd count=10 if=/dev/urandom conv=notrunc iflag=fullblock of=k
>>
>> However, when we try to copy "k", we get a file, "k2" of the
>> expected size, but with only NUL bytes for contents:
>
> So the extent info is not updated until sync(),
> which means cp will consider the "unwritten" extent
> as NUL data and not bother to read it :(
>
> I guess this is a corner case that was missed
> in the fixups for ext4 (and btrfs?) in 2.6.38 for this?
> I've copied ext4 devs for clarification.
> I.E. if you do this:
>
> fallocate -l 10000000 -n k
> dd count=10 if=/dev/urandom conv=notrunc iflag=fullblock of=k
> filefrag -v k
>
> Do you get all extents still unwritten.
> I do on my 2.6.35.11-83.fc14.i686 kernel, but I expected that.

Right. The new extent remains unwritten until after a sync
with ext4 on both Fedora 15 and rawhide kernels.

Here's F15:

$ fallocate -l 10000000 -n k
$ dd count=10 if=/dev/urandom conv=notrunc iflag=fullblock of=k
...
$ filefrag -v k
Filesystem type is: ef53
File size of k is 5120 (2 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 6498304 2442 unwritten,eof
k: 1 extent found
$ sync
$ filefrag -v k
Filesystem type is: ef53
File size of k is 5120 (2 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 6498304 2 eof
1 2 6498306 2440 unwritten,eof
k: 1 extent found

Same problem with rawhide, but slightly different layout:

$ fallocate -l 10000000 -n k
$ dd count=10 if=/dev/urandom conv=notrunc iflag=fullblock of=k
...
$ filefrag -v k
Filesystem type is: ef53
File size of k is 5120 (2 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 2054144 2048 unwritten,eof
1 2048 2060288 2056191 394 unwritten,eof
k: 2 extents found
$ filefrag -v k
[Exit 1]
$ sync
$ filefrag -v k
Filesystem type is: ef53
File size of k is 5120 (2 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 2054144 2 eof
1 2 2054146 2046 unwritten,eof
2 2048 2060288 2056191 394 unwritten,eof
k: 2 extents found