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Subject[tip: x86/iopl] x86/ioperm: Share I/O bitmap if identical
The following commit has been merged into the x86/iopl branch of tip:

Commit-ID: b9027e7c44922bf1499adb76f87da7a374a48c22
Gitweb: https://git.kernel.org/tip/b9027e7c44922bf1499adb76f87da7a374a48c22
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Wed, 13 Nov 2019 21:42:54 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 14 Nov 2019 20:15:05 +01:00

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 <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191113210105.078437077@linutronix.de


---
arch/x86/include/asm/io_bitmap.h | 5 +++-
arch/x86/kernel/ioport.c | 48 +++++++++++++++++++++++++------
arch/x86/kernel/process.c | 39 +++----------------------
3 files changed, 50 insertions(+), 42 deletions(-)

diff --git a/arch/x86/include/asm/io_bitmap.h b/arch/x86/include/asm/io_bitmap.h
index 784a88e..b664baa 100644
--- a/arch/x86/include/asm/io_bitmap.h
+++ b/arch/x86/include/asm/io_bitmap.h
@@ -2,15 +2,20 @@
#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);
diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index f9fc69a..f82ca1c 100644
--- 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)
preempt_disable();
tss_update_io_bitmap();
preempt_enable();
- kfree(iobm);
+ if (iobm && refcount_dec_and_test(&iobm->refcnt))
+ kfree(iobm);
}

/*
@@ -58,9 +70,32 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
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
* exit to user mode. So this needs no protection.
*/
@@ -86,16 +121,11 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_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;
}
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 75f8b13..bfc935d 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -122,37 +122,13 @@ static int set_new_tls(struct task_struct *p, unsigned long tls)
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);
@@ -193,16 +169,13 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
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;
}

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