zfsonlinux/zfs — pull request #1865 by behlendorf (2013-11-13 22:54:55+00:00)
Title: Reduce stack for traverse_visitbp() recursion

During pool import stack overflows may still occur due to the
potentially deep recursion of traverse_visitbp(). This is most
likely to occur when additional layers are added to the block
device stack such as DM multipath. To minimize the stack usage
for this call path the following changes were made:

1) Added the keywork 'noinline' to the vdev_*_map_alloc() functions
to prevent them from being inlined by gcc. This reduced the
stack usage of vdev_raidz_io_start() from 208 to 128 bytes, and
vdev_mirror_io_start() from 144 to 128 bytes.

2) The 'saved_poolname' charater array in zfsdev_ioctl() was moved
from the stack to the heap. This reduced the stack usage of
zfsdev_ioctl() from 368 to 112 bytes.

3) The major saving came from slimming down traverse_visitbp() from
from 224 to 144 bytes. Since this function is called recursively
the 80 bytes saved per invokation adds up. The following changes
were made:

a) The 'hard' local variable was replaced by a TD_HARD() macro.

b) The 'pd' local variable was replaced by 'td->td_pfd' references.

c) The zbookmark_t was moved to the heap. This does cost us an
additional memory allocation per recursion by that cost should
still be minimal. The cost could be further reduced by adding
a dedicated zbookmark_t slab cache.

d) The variable declarations in 'if (BP_GET_LEVEL()) { }' were
restructured to use the minimum amount of stack. This includes
removing the 'cbp' local variable.

Overall for the offending use case roughly 1584 of total stack space
has been saved. This is enough to avoid overflowing the stack on
stock kernels with 8k stacks. See #1778 for additional details.

Signed-off-by: Brian Behlendorf
Issue #1778