On Thu, 16 Feb 2012, Ted Ts'o wrote:
> On Thu, Feb 16, 2012 at 06:10:57PM -0500, Phillip Susi wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > On 02/16/2012 05:58 PM, Ted Ts'o wrote:
> > > I don't see the bug here. If there are no leftover sparse bytes,
> > > there's no need to write the last zero byte. The whole point was to
> > > make sure i_size was set correctly, and if sparse==0, then i_size is
> > > correct.
> >
> > - From what I can see, when sparse == 0, the last write does a seek
> > to move the file pointer, but doesn't write anything beyond the last
> > hole, so i_size is not updated. This resulted in an image file I
> > took of a 20gb fs being 124 MiB too small. I can only assume that
> > this is to be expected, and is the reason for passing one byte of
> > zero_buff to write_block instead of not giving it any bytes to
> > write, and just asking it to do the seek the way the loop does.
>
> Sorry, I'm still not understanding what you're concerned about. The
> last write should seek to the end of the file system, and write a
> single byte --- which would be past the last hole. The goal is to
> make sure the file system is large enough that e2fsck doesn't
> complain about the file system image being apparently too small.
>
> And in fact, it's doing the right thing:
>
> tytso.root@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> {/home/tytso}
> 2007# strace -o /tmp/foo /sbin/e2image -r /dev/funarg/library /kbuild/test.img
> e2image 1.42 (29-Nov-2011)
> <tytso.root@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> {/home/tytso}
> 2008# tail /tmp/foo
> lseek(5, 1048576, SEEK_CUR) = 16102031360
> lseek(5, 1048576, SEEK_CUR) = 16103079936
> lseek(5, 1048576, SEEK_CUR) = 16104128512
> lseek(5, 1048576, SEEK_CUR) = 16105177088
^^^^^^^^^^^^^^^^^^^^^^^^^^^
So if blocks count ends right here^^, then the last write would not
happen, because sparse == 0. The reason is that we will seek when the
sparse >= 1024*1024 and then set sparse = 0
if (sparse >= 1024*1024) {
write_block(fd, 0, sparse, 0, 0);
sparse = 0;
}
if the file system ends right after that point, then we will not write
that one last byte.
We can easily fix that by doing this instead:
if (sparse > 1024*1024) {
write_block(fd, 0, 1024*1024, 0, 0);
sparse -= 1024*1024;
}
Thanks!
-Lukas
> lseek(5, 950271, SEEK_CUR) = 16106127359
> write(5, "\0", 1) = 1 <=====
> munmap(0x7f7b6dace000, 495616) = 0
> close(4) = 0
> close(3) = 0
> exit_group(0) = ?
>
> I don't understand why you're saying that it's not writing anything
> beyond the last hole, and why you're saying i_size is not being
> updated. It's working for me; I can run e2fsck on the generated
> image, and it's not complaining that the file system is too small.
>
> - Ted
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