On Tue, Dec 25, 2018 at 04:10:25PM -0800, Liu Bo wrote:

I had a patch[1] to address the problem but with adding reservation
only when we need to (in ext4_ext_try_to_merge_up).

Sorry, I had forgotten that you had that patch pending.  I believe
that merging up isn't the only case we need to worry about, though.
For example, if we write into the middle of a larger unwritten region,
we might havec to change a single region into three regions, which
might require a node split --- and in that case, it's not optional, as
would be in the try_to_merge_up situation.  If we don't have enough
credits to do a node split our only choices would be mark the file
system as corrupted, or to fail the write with ENOSPC (which would be
very confusing to the user/application).

Cheers,

					- Ted