hugetlbfs: fix hugetlb page migration/fault race causing SIGBUS

Li Wang discovered that LTP/move_page12 V2 sometimes triggers SIGBUS in
the kernel-v5.2.3 testing.  This is caused by a race between hugetlb
page migration and page fault.

If a hugetlb page can not be allocated to satisfy a page fault, the task
is sent SIGBUS.  This is normal hugetlbfs behavior.  A hugetlb fault
mutex exists to prevent two tasks from trying to instantiate the same
page.  This protects against the situation where there is only one
hugetlb page, and both tasks would try to allocate.  Without the mutex,
one would fail and SIGBUS even though the other fault would be
successful.

There is a similar race between hugetlb page migration and fault.
Migration code will allocate a page for the target of the migration.  It
will then unmap the original page from all page tables.  It does this
unmap by first clearing the pte and then writing a migration entry.  The
page table lock is held for the duration of this clear and write
operation.  However, the beginnings of the hugetlb page fault code
optimistically checks the pte without taking the page table lock.  If
clear (as it can be during the migration unmap operation), a hugetlb
page allocation is attempted to satisfy the fault.  Note that the page
which will eventually satisfy this fault was already allocated by the
migration code.  However, the allocation within the fault path could
fail which would result in the task incorrectly being sent SIGBUS.

Ideally, we could take the hugetlb fault mutex in the migration code
when modifying the page tables.  However, locks must be taken in the
order of hugetlb fault mutex, page lock, page table lock.  This would
require significant rework of the migration code.  Instead, the issue is
addressed in the hugetlb fault code.  After failing to allocate a huge
page, take the page table lock and check for huge_pte_none before
returning an error.  This is the same check that must be made further in
the code even if page allocation is successful.

Link: http://lkml.kernel.org/r/20190808000533.7701-1-mike.kravetz@oracle.com
Fixes: 290408d4a250 ("hugetlb: hugepage migration core")
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reported-by: Li Wang <liwang@redhat.com>
Tested-by: Li Wang <liwang@redhat.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Cyril Hrubis <chrubis@suse.cz>
Cc: Xishi Qiu <xishi.qiuxishi@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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