[patch V2 11/16] x86/ioperm: Share I/O bitmap if identical

[Thomas Gleixner]

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 <tglx@xxxxxxxxxxxxx>
---
V2: New patch
---
 arch/x86/include/asm/iobitmap.h |    5 +++++
 arch/x86/kernel/ioport.c        |   38 ++++++++++++++++++++++++++++++++------
 arch/x86/kernel/process.c       |   39 ++++++---------------------------------
 3 files changed, 43 insertions(+), 39 deletions(-)

--- a/arch/x86/include/asm/iobitmap.h
+++ b/arch/x86/include/asm/iobitmap.h
@@ -2,10 +2,12 @@
 #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;
 	unsigned int		io_bitmap_max;
 	union {
 		unsigned long	bits[IO_BITMAP_LONGS];
@@ -13,6 +15,9 @@ struct io_bitmap {
 	};
 };
 
+struct task_struct;
+
+void io_bitmap_share(struct task_struct *tsk);
 void io_bitmap_exit(void);
 
 void tss_update_io_bitmap(void);
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -16,6 +16,17 @@
 
 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;
@@ -25,7 +36,8 @@ void io_bitmap_exit(void)
 	clear_thread_flag(TIF_IO_BITMAP);
 	tss_update_io_bitmap();
 	preempt_enable();
-	kfree(iobm);
+	if (iobm && refcount_dec_and_test(&iobm->refcnt))
+		kfree(iobm);
 }
 
 /*
@@ -59,8 +71,26 @@ long ksys_ioperm(unsigned long from, uns
 			return -ENOMEM;
 
 		memset(iobm->bits, 0xff, sizeof(iobm->bits));
+		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;
+		io_bitmap_exit();
 	}
 
+	/* Set the tasks io_bitmap pointer (might be the same) */
+	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
 	 * exit to user mode. So this needs no protection.
@@ -88,12 +118,8 @@ long ksys_ioperm(unsigned long from, uns
 
 	iobm->io_bitmap_max = (max_long + 1) * sizeof(unsigned long);
 
-	/* Update the sequence number to force an update in switch_to() */
+	/* Increment the sequence number to force a TSS update */
 	iobm->sequence = atomic64_add_return(1, &io_bitmap_sequence);
-	/* Set the tasks io_bitmap pointer (might be the same) */
-	t->io_bitmap = iobm;
-	/* Mark it active for context switching and exit to user mode */
-	set_thread_flag(TIF_IO_BITMAP);
 
 	return 0;
 }
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -122,37 +122,13 @@ static int set_new_tls(struct task_struc
 		return do_set_thread_area_64(p, ARCH_SET_FS, tls);
 }
 
-static inline int copy_io_bitmap(struct task_struct *tsk)
-{
-	struct io_bitmap *bm = current->thread.io_bitmap;
-
-	if (likely(!test_tsk_thread_flag(current, TIF_IO_BITMAP)))
-		return 0;
-
-	tsk->thread.io_bitmap = kmemdup(bm, sizeof(*bm), 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);
@@ -193,16 +169,13 @@ int copy_thread_tls(unsigned long clone_
 	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;
 }