Partly revert "mm/thp: carry over dirty bit when thp splits on pmd"

Anatoly Pugachev reported sparc64 breakage on the patch:

https://lore.kernel.org/r/[email protected]

The sparc64 impl of pte_mkdirty() is definitely slightly special in that
it leverages a code patching mechanism for sun4u/sun4v on relevant pgtable
entry operations.

Before having a clue of why the sparc64 is special and caused the patch to
SIGSEGV the processes, revert the patch for now.  The swap path of dirty
bit inheritage is kept because that's using the swap shared code so we
assume it'll not be affected.

Link: https://lkml.kernel.org/r/Y1Wbi4yyVvDtg4zN@x1n
Fixes: 0ccf7f1 ("mm/thp: carry over dirty bit when thp splits on pmd")
Signed-off-by: Peter Xu <[email protected]>
Reported-by: Anatoly Pugachev <[email protected]> 
Tested-by: Anatoly Pugachev <[email protected]> 
Cc: Alistair Popple <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: xzpeter

mm/huge_memory.c

@@ -2206,9 +2206,12 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 				entry = pte_wrprotect(entry);
 			if (!young)
 				entry = pte_mkold(entry);
-			/* NOTE: this may set soft-dirty too on some archs */
-			if (dirty)
-				entry = pte_mkdirty(entry);
+			/*
+			 * NOTE: we don't do pte_mkdirty when dirty==true
+			 * because it breaks sparc64 which can sigsegv
+			 * random process.  Need to revisit when we figure
+			 * out what is special with sparc64.
+			 */
 			if (soft_dirty)
 				entry = pte_mksoft_dirty(entry);
 			if (uffd_wp)