[PATCH] ext2/3 file limits to avoid overflowing i_blocks

As discussed before, we can overflow i_blocks in ext2/ext3 inodes by
growing a file up to 2TB.  That gives us 2^32 sectors of data in the file;
but once you add on the indirect tree and possible EA/ACL metadata,
i_blocks will wrap beyond 2^32.  Consensus seemed to be that the best way
to avoid this was simply to stop files getting so large that this was a
problem in the first place; anything else would lead to complications if a
sparse file tried to overflow that 2^32 sector limit while filling in
holes.

I wrote a small program to calculate the total indirect tree overhead for
any given file size, and 0x1ff7fffe000 turned out to be the largest file we
can get without the total i_blocks overflowing 2^32.

But in testing, that *just* wrapped --- we need to limit the file to be one
page smaller than that to deal with the possibility of an EA/ACL block
being accounted against i_blocks.

So this patch has been tested, at least on ext3, by letting a file grow
densely to its maximum size permitted by the kernel; at 0x1ff7fffe000, stat
shows the file to have wrapped back exactly to 0 st_blocks, but with the
limit at 0x1ff7fffd000, du shows it occupying the expected 2TB-blocksize
bytes.

Signed-off-by: Stephen Tweedie <sct@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

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