[PATCH] xfs: fix allocation length overflow in xfs_bmapi_write()

[Ben Myers]

On Thu, Dec 01, 2011 at 10:24:20PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> When testing the new xfstests --large-fs option that does very large
> file preallocations, this assert was tripped deep in
> xfs_alloc_vextent():
> 
> XFS: Assertion failed: args->minlen <= args->maxlen, file: fs/xfs/xfs_alloc.c, line: 2239
> 
> The allocation was trying to allocate a zero length extent because
> the lower 32 bits of the allocation length was zero. The remaining
> length of the allocation to be done was an exact multiple of 2^32 -
> the first case I saw was at 496TB remaining to be allocated.
> 
> This turns out to be an overflow when converting the allocation
> length (a 64 bit quantity) into the extent length to allocate (a 32
> bit quantity), and it requires the length to be allocated an exact
> multiple of 2^32 blocks to trip the assert.
> 
> Fix it by limiting the extent lenth to allocate to MAXEXTLEN.
> 
> Signed-off-by: Dave Chinner <dchinner at redhat.com>

Looks good to me.
Reviewed-by: Ben Myers <bpm at sgi.com>