mm/memory: remove page fault assumption of compound page size

A compound page in the page cache will not necessarily be of PMD size,
so check explicitly.

[willy@infradead.org: fix remove page fault assumption of compound page size]
  Link: https://lkml.kernel.org/r/20201001152259.14932-1-willy@infradead.orgSigned-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lkml.kernel.org/r/20200908195539.25896-3-willy@infradead.orgSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -3709,13 +3709,14 @@ static vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
 	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
 	pmd_t entry;
 	int i;
-	vm_fault_t ret;
+	vm_fault_t ret = VM_FAULT_FALLBACK;
 
 	if (!transhuge_vma_suitable(vma, haddr))
-		return VM_FAULT_FALLBACK;
+		return ret;
 
-	ret = VM_FAULT_FALLBACK;
 	page = compound_head(page);
+	if (compound_order(page) != HPAGE_PMD_ORDER)
+		return ret;
 
 	/*
 	 * Archs like ppc64 need additonal space to store information