WARN_ON in move_normal_pmd

[Michal Hocko]

Hi,
our QA is regularly hitting
[  544.198822][T20518] WARNING: CPU: 1 PID: 20518 at ../mm/mremap.c:255 move_pgt_entry+0x4c6/0x510
triggered by thp01 LTP test. This has been brought up in the past and
resulted in f81fdd0c4ab7 ("mm: document warning in move_normal_pmd() and
make it warn only once"). While it is good that the underlying problem
is understood, it doesn't seem there is enough interest to fix it
properly. Which is fair but I am wondering whether the WARN_ON gives
us anything here.

Our QA process collects all unexpected side effects of tests and a WARN*
in the log is certainly one of those. This trigger bugs which are mostly
ignored as there is no upstream fix for them. This alone is nothing
really critical but there are workloads which do run with panic on warn
configured and this issue would put the system down which is unnecessary
IMHO. Would it be sufficient to replace the WARN_ON_ONCE by
pr_warn_once?

Thanks!
-- 
Michal Hocko
SUSE Labs