Use of kmalloc vs vmalloc in ext4?

[Curt Wohlgemuth <curtw@google.com>]

I've been running various tests of ext4 partitions lately, and have
found that with very low memory situations, I'm getting intermittent
mount failures due to ENOMEM from ext4_mb_init() and
ext4_fill_flex_info() .  Here's a typical dmesg from the latter:

          EXT4-fs: not enough memory for 8198 flex groups
          EXT4-fs: unable to initialize flex_bg meta info!

This is from a kzalloc() call of size ~64k .  I think the
ext4_mb_init() calls to kmalloc() and alloc_percpu() are even smaller.

I was wondering why all the code in ext4 (and ext[23], for that
matter) uses kmalloc() and friends instead of vmalloc(), at least
where it's safe; is it just for performance reasons?

I've seen the above errors when I do a mount -a, causing several
partitions to be mounted; I can usually mount the failed ones by hand
right afterwards, but this is a big difference for us, in our
environment, compared to, say, ext2 partitions.

Thanks,
Curt