generic: test writev with page fault when it processes iov

generic: test writev with page fault when it processes iov

We met a kernel assertion failure recently as below:

  XFS: Assertion failed: tp->t_blk_res_used <= tp->t_blk_res, file: fs/xfs/xfs_trans.c, line: 309

Eric Sandeen digged into it and find a good reproducer.

The problem comes when the several IO vectors are copied in, and
it runs into page faults, which stops the copy before all vectors
are copied. XFS sees this as a failed/short write, and so tries
to unmap the blocks & truncate away the pages in xfs_vm_write_end.

generic_perform_write is looping, and comes back around for the
other iovecs, but the page is still there, the buffer head is still
mapped, and so a new delalloc block isn't allocated - and ends up
being allocated at writeback time, despite the fact that we "should"
have accounted for it all at delalloc write time, and trips the
assert.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>