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.com
Signed-off-by: Song Liu <songliubraving@fb.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus 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 @@ retry:
 				/* 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 @@ put_old:
 	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;
 }