Re: [PATCH] ext4: use vmtruncate() instead of ext4_truncate() in ext4_setattr()

[Eric Sandeen <sandeen@redhat.com>]

On 5/18/11 1:13 AM, Dave Chinner wrote:

> It's entirely up to the filesystem how it treats blocks beyond EOF
> during truncation. XFS frees them on truncate down, because it is
> much safer to just truncate away everything beyond the new EOF than
> to leave written extents beyond EOF as potential landmines.
> 
> Indeed, that's why calling vmtruncate() as a bad fix. If you have:
> 
> 
> 	       NUUUUUUUUUUWWWWWWWWWOUUUUUUUUU
>        ....----+----------+--------+--------+
>                A	  B        C        D
> 
> Where	A = new EOF (N)
> 	A->B = unwritten (U)
> 	B->C = written (W)
> 	C = old EOF (O)
> 	C->D = unwritten (U)
> 
> Then just calling vmtruncate() will leave the blocks in the range
> B->C as written blocks. Hence then doing an extending truncate back
> out to D will expose stale data rather than zeros in the range
> B->C....

Hm, running recent xfstests which includes fsx-with-fallocate should probably
eventually catch that then.

-Eric
 
> Cheers,
> 
> Dave.