x86/ioperm: Share I/O bitmap if identical

The I/O bitmap is duplicated on fork. That's wasting memory and slows down
fork. There is no point to do so. As long as the bitmap is not modified it
can be shared between threads and processes.

Add a refcount and just share it on fork. If a task modifies the bitmap
then it has to do the duplication if and only if it is shared.

Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Andy Lutomirski <[e-mail]>

arch/x86/include/asm/io_bitmap.h

···
 #ifndef _ASM_X86_IOBITMAP_H
 #define _ASM_X86_IOBITMAP_H
 
+#include <linux/refcount.h>
 #include <asm/processor.h>
 
 struct io_bitmap {
 	u64		sequence;
+	refcount_t	refcnt;
 	/* The maximum number of bytes to copy so all zero bits are covered */
 	unsigned int	max;
 	unsigned long	bitmap[IO_BITMAP_LONGS];
 };
 
+struct task_struct;
+
+void io_bitmap_share(struct task_struct *tsk);
 void io_bitmap_exit(void);
 
 void tss_update_io_bitmap(void);

arch/x86/kernel/ioport.c

···
 
 static atomic64_t io_bitmap_sequence;
 
+void io_bitmap_share(struct task_struct *tsk)
+ {
+	/*
+	 * Take a refcount on current's bitmap. It can be used by
+	 * both tasks as long as none of them changes the bitmap.
+	 */
+	refcount_inc(&current->thread.io_bitmap->refcnt);
+	tsk->thread.io_bitmap = current->thread.io_bitmap;
+	set_tsk_thread_flag(tsk, TIF_IO_BITMAP);
+}
+
 void io_bitmap_exit(void)
 {
 	struct io_bitmap *iobm = current->thread.io_bitmap;
···
 	preempt_disable();
 	tss_update_io_bitmap();
 	preempt_enable();
-	kfree(iobm);
+	if (iobm && refcount_dec_and_test(&iobm->refcnt))
+		kfree(iobm);
 }
 
 /*
···
 			return -ENOMEM;
 
 		memset(iobm->bitmap, 0xff, sizeof(iobm->bitmap));
+		refcount_set(&iobm->refcnt, 1);
 	}
+
+	/*
+	 * If the bitmap is not shared, then nothing can take a refcount as
+	 * current can obviously not fork at the same time. If it's shared
+	 * duplicate it and drop the refcount on the original one.
+	 */
+	if (refcount_read(&iobm->refcnt) > 1) {
+		iobm = kmemdup(iobm, sizeof(*iobm), GFP_KERNEL);
+		if (!iobm)
+			return -ENOMEM;
+		refcount_set(&iobm->refcnt, 1);
+		io_bitmap_exit();
+	}
+
+	/*
+	 * Store the bitmap pointer (might be the same if the task already
+	 * head one). Must be done here so freeing the bitmap when all
+	 * permissions are dropped has the pointer set up.
+	 */
+	t->io_bitmap = iobm;
+	/* Mark it active for context switching and exit to user mode */
+	set_thread_flag(TIF_IO_BITMAP);
 
 	/*
 	 * Update the tasks bitmap. The update of the TSS bitmap happens on
···
 
 	iobm->max = (max_long + 1) * sizeof(unsigned long);
 
-	/* Update the sequence number to force an update in switch_to() */
-	iobm->sequence = atomic64_add_return(1, &io_bitmap_sequence);
-
 	/*
-	 * Store the bitmap pointer (might be the same if the task already
-	 * head one). Set the TIF flag, just in case this is the first
-	 * invocation.
+	 * Update the sequence number to force a TSS update on return to
+	 * user mode.
 	 */
-	t->io_bitmap = iobm;
-	set_thread_flag(TIF_IO_BITMAP);
+	iobm->sequence = atomic64_add_return(1, &io_bitmap_sequence);
 
 	return 0;
 }

arch/x86/kernel/process.c

···
 		return do_set_thread_area_64(p, ARCH_SET_FS, tls);
 }
 
-static inline int copy_io_bitmap(struct task_struct *tsk)
-{
-	struct io_bitmap *iobm = current->thread.io_bitmap;
-
-	if (likely(!test_tsk_thread_flag(current, TIF_IO_BITMAP)))
-		return 0;
-
-	tsk->thread.io_bitmap = kmemdup(iobm, sizeof(*iobm), GFP_KERNEL);
-
-	if (!tsk->thread.io_bitmap)
-		return -ENOMEM;
-
-	set_tsk_thread_flag(tsk, TIF_IO_BITMAP);
-	return 0;
-}
-
-static inline void free_io_bitmap(struct task_struct *tsk)
-{
-	if (tsk->thread.io_bitmap) {
-		kfree(tsk->thread.io_bitmap);
-		tsk->thread.io_bitmap = NULL;
-	}
-}
-
 int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 		    unsigned long arg, struct task_struct *p, unsigned long tls)
 {
 	struct inactive_task_frame *frame;
 	struct fork_frame *fork_frame;
 	struct pt_regs *childregs;
-	int ret;
+	int ret = 0;
 
 	childregs = task_pt_regs(p);
 	fork_frame = container_of(childregs, struct fork_frame, regs);
···
 	task_user_gs(p) = get_user_gs(current_pt_regs());
 #endif
 
-	ret = copy_io_bitmap(p);
-	if (ret)
-		return ret;
-
 	/* Set a new TLS for the child thread? */
-	if (clone_flags & CLONE_SETTLS) {
+	if (clone_flags & CLONE_SETTLS)
 		ret = set_new_tls(p, tls);
-		if (ret)
-			free_io_bitmap(p);
-	}
+
+	if (!ret && unlikely(test_tsk_thread_flag(current, TIF_IO_BITMAP)))
+		io_bitmap_share(p);
+
 	return ret;
 }