[BUG] new copy_hugetlb_page_range() causing crashes

[Guillaume Morin]

Naoya, Hugh,

I am seeing lots of crashes with the new copy_hugetlb_page_range() code
added by 4a705fef986231a3e7a6b1a6d3c37025f021f49f for some set of
programs.

Specifically, I am running some test programs which use huge pages for
malloc (through libhugetlbfs with HUGETLB_MORECORE=yes and
HUGETLB_NO_PREFAULT= set) that also do fork() a lot.  The crashes are
very diverse: assertion failures in malloc() or the python GC code, some
SIGSEGV as well.

I started observing these crashes with 3.4.98 and 3.14.12 which just got
a backport of the patch above (as
2bcdd4933ff4dc46445dcae93cb37c648283b782 in the stable branch).  The 3.4
and 3.14 patches are identical to the upstream commit so that's not a patch
backport issue.

If I revert only 2bcdd4933ff4dc46445dcae93cb37c648283b782 in my 3.4
tree, the crashes disappear right away and everything is stable. 

-- 
Guillaume Morin <guillaume@xxxxxxxxxxx>
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