uprobe: collapse THP pmd after removing all uprobes

After all uprobes are removed from the huge page (with PTE pgtable), it is
possible to collapse the pmd and benefit from THP again.  This patch does
the collapse by calling collapse_pte_mapped_thp().

Link: http://lkml.kernel.org/r/20190815164525.1848545-7-songliubraving@fb.comSigned-off-by: NSong Liu <songliubraving@fb.com>
Acked-by: NKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Nkbuild test robot <lkp@intel.com>
Reviewed-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/events/uprobes.c

@@ -26,6 +26,7 @@
 #include <linux/percpu-rwsem.h>
 #include <linux/task_work.h>
 #include <linux/shmem_fs.h>
+#include <linux/khugepaged.h>
 
 #include <linux/uprobes.h>
 
@@ -472,6 +473,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	struct page *old_page, *new_page;
 	struct vm_area_struct *vma;
 	int ret, is_register, ref_ctr_updated = 0;
+	bool orig_page_huge = false;
 
 	is_register = is_swbp_insn(&opcode);
 	uprobe = container_of(auprobe, struct uprobe, arch);
@@ -529,6 +531,9 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 				/* let go new_page */
 				put_page(new_page);
 				new_page = NULL;
+
+				if (PageCompound(orig_page))
+					orig_page_huge = true;
 			}
 			put_page(orig_page);
 		}
@@ -547,6 +552,10 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	if (ret && is_register && ref_ctr_updated)
 		update_ref_ctr(uprobe, mm, -1);
 
+	/* try collapse pmd for compound page */
+	if (!ret && orig_page_huge)
+		collapse_pte_mapped_thp(mm, vaddr);
+
 	return ret;
 }