mm/ksm: fix interaction with THP

This patch fixes a corner case for KSM.  When two pages belong or
belonged to the same transparent hugepage, and they should be merged,
KSM fails to split the page, and therefore no merging happens.

This bug can be reproduced by:
* making sure ksm is running (in case disabling ksmtuned)
* enabling transparent hugepages
* allocating a THP-aligned 1-THP-sized buffer
  e.g. on amd64: posix_memalign(&p, 1<<21, 1<<21)
* filling it with the same values
  e.g. memset(p, 42, 1<<21)
* performing madvise to make it mergeable
  e.g. madvise(p, 1<<21, MADV_MERGEABLE)
* waiting for KSM to perform a few scans

The expected outcome is that the all the pages get merged (1 shared and
the rest sharing); the actual outcome is that no pages get merged (1
unshared and the rest volatile)

The reason of this behaviour is that we increase the reference count
once for both pages we want to merge, but if they belong to the same
hugepage (or compound page), the reference counter used in both cases is
the one of the head of the compound page.  This means that
split_huge_page will find a value of the reference counter too high and
will fail.

This patch solves this problem by testing if the two pages to merge
belong to the same hugepage when attempting to merge them.  If so, the
hugepage is split safely.  This means that the hugepage is not split if
not necessary.

Link: http://lkml.kernel.org/r/1521548069-24758-1-git-send-email-imbrenda@linux.vnet.ibm.com


Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Co-authored-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>