On Tue, 20 Sep 2016 10:37:04 -0700
Mike Kravetz [off-list ref] wrote:

Cc'ed Rui Teng and Dave Hansen as they were discussing the issue in
this thread:
https://lkml.org/lkml/2016/9/13/146

Ah, thanks, I didn't see that.

Their approach (I believe) would be to fail the offline operation in
this case.  However, I could argue that failing the operation, or
dissolving the unused huge page containing the area to be offlined is
the right thing to do.

I never thought too much about the VM_BUG_ON(), but you are correct in
that it should be removed in either case.

The other thing that needs to be changed is the locking in
dissolve_free_huge_page().  I believe the lock only needs to be held if
we are removing the huge page from the pool.  It is not a correctness
but performance issue.

Yes, that looks odd, that's why in my patch I moved the PageHuge() check
out from dissolve_free_huge_page(), up into the loop in
dissolve_free_huge_pages(). This way dissolve_free_huge_page() with its
locking should only be called once per memory block, in the case where
this memory block is part of a gigantic hugepage.

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