diff mbox series

[net] vhost: don't use kmap() to log dirty pages

Message ID 1557725265-63525-1-git-send-email-jasowang@redhat.com
State New, archived
Headers show
Series [net] vhost: don't use kmap() to log dirty pages | expand

Commit Message

Jason Wang May 13, 2019, 5:27 a.m. UTC
Vhost log dirty pages directly to a userspace bitmap through GUP and
kmap_atomic() since kernel doesn't have a set_bit_to_user()
helper. This will cause issues for the arch that has virtually tagged
caches. The way to fix is to keep using userspace virtual
address. Fortunately, futex has arch_futex_atomic_op_inuser() which
could be used for setting a bit to user.

Note there're several cases that futex helper can fail e.g a page
fault or the arch that doesn't have the support. For those cases, a
simplified get_user()/put_user() pair protected by a global mutex is
provided as a fallback. The fallback may lead false positive that
userspace may see more dirty pages.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhart@infradead.org>
Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
Changes from RFC V2:
- drop GUP and provide get_user()/put_user() fallbacks
- round down log_base
Changes from RFC V1:
- switch to use arch_futex_atomic_op_inuser()
---
 drivers/vhost/vhost.c | 54 ++++++++++++++++++++++++++++-----------------------
 1 file changed, 30 insertions(+), 24 deletions(-)

Comments

David Miller May 13, 2019, 4:42 p.m. UTC | #1
From: Jason Wang <jasowang@redhat.com>
Date: Mon, 13 May 2019 01:27:45 -0400

> Vhost log dirty pages directly to a userspace bitmap through GUP and
> kmap_atomic() since kernel doesn't have a set_bit_to_user()
> helper. This will cause issues for the arch that has virtually tagged
> caches. The way to fix is to keep using userspace virtual
> address. Fortunately, futex has arch_futex_atomic_op_inuser() which
> could be used for setting a bit to user.
> 
> Note there're several cases that futex helper can fail e.g a page
> fault or the arch that doesn't have the support. For those cases, a
> simplified get_user()/put_user() pair protected by a global mutex is
> provided as a fallback. The fallback may lead false positive that
> userspace may see more dirty pages.
> 
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Darren Hart <dvhart@infradead.org>
> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
> Signed-off-by: Jason Wang <jasowang@redhat.com>

I want to see a review from Michael for this change before applying.
Jason Wang May 14, 2019, 3:42 a.m. UTC | #2
On 2019/5/14 上午12:42, David Miller wrote:
> From: Jason Wang <jasowang@redhat.com>
> Date: Mon, 13 May 2019 01:27:45 -0400
>
>> Vhost log dirty pages directly to a userspace bitmap through GUP and
>> kmap_atomic() since kernel doesn't have a set_bit_to_user()
>> helper. This will cause issues for the arch that has virtually tagged
>> caches. The way to fix is to keep using userspace virtual
>> address. Fortunately, futex has arch_futex_atomic_op_inuser() which
>> could be used for setting a bit to user.
>>
>> Note there're several cases that futex helper can fail e.g a page
>> fault or the arch that doesn't have the support. For those cases, a
>> simplified get_user()/put_user() pair protected by a global mutex is
>> provided as a fallback. The fallback may lead false positive that
>> userspace may see more dirty pages.
>>
>> Cc: Christoph Hellwig <hch@infradead.org>
>> Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Darren Hart <dvhart@infradead.org>
>> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> I want to see a review from Michael for this change before applying.


No problem, since kbuild spotted an issue. Let me post V2.

Thanks
Michael S. Tsirkin May 14, 2019, 10:16 p.m. UTC | #3
On Mon, May 13, 2019 at 01:27:45AM -0400, Jason Wang wrote:
> Vhost log dirty pages directly to a userspace bitmap through GUP and
> kmap_atomic() since kernel doesn't have a set_bit_to_user()
> helper. This will cause issues for the arch that has virtually tagged
> caches. The way to fix is to keep using userspace virtual
> address. Fortunately, futex has arch_futex_atomic_op_inuser() which
> could be used for setting a bit to user.
> 
> Note there're several cases that futex helper can fail e.g a page
> fault or the arch that doesn't have the support. For those cases, a
> simplified get_user()/put_user() pair protected by a global mutex is
> provided as a fallback. The fallback may lead false positive that
> userspace may see more dirty pages.
> 
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Darren Hart <dvhart@infradead.org>
> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
> Changes from RFC V2:
> - drop GUP and provide get_user()/put_user() fallbacks
> - round down log_base
> Changes from RFC V1:
> - switch to use arch_futex_atomic_op_inuser()
> ---
>  drivers/vhost/vhost.c | 54 ++++++++++++++++++++++++++++-----------------------
>  1 file changed, 30 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 351af88..7fa05ba 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -31,6 +31,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/interval_tree_generic.h>
>  #include <linux/nospec.h>
> +#include <asm/futex.h>
>  
>  #include "vhost.h"
>  
> @@ -43,6 +44,8 @@
>  MODULE_PARM_DESC(max_iotlb_entries,
>  	"Maximum number of iotlb entries. (default: 2048)");
>  
> +static DEFINE_MUTEX(vhost_log_lock);
> +
>  enum {
>  	VHOST_MEMORY_F_LOG = 0x1,
>  };
> @@ -1692,28 +1695,31 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>  }
>  EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
>  
> -/* TODO: This is really inefficient.  We need something like get_user()
> - * (instruction directly accesses the data, with an exception table entry
> - * returning -EFAULT). See Documentation/x86/exception-tables.txt.
> - */
> -static int set_bit_to_user(int nr, void __user *addr)
> +static int set_bit_to_user(int nr, u32 __user *addr)
>  {
> -	unsigned long log = (unsigned long)addr;
> -	struct page *page;
> -	void *base;
> -	int bit = nr + (log % PAGE_SIZE) * 8;
> +	u32 old;
>  	int r;
>  
> -	r = get_user_pages_fast(log, 1, 1, &page);
> -	if (r < 0)
> -		return r;
> -	BUG_ON(r != 1);
> -	base = kmap_atomic(page);
> -	set_bit(bit, base);
> -	kunmap_atomic(base);
> -	set_page_dirty_lock(page);
> -	put_page(page);
> +	r = arch_futex_atomic_op_inuser(FUTEX_OP_OR, 1 << nr, &old, addr);
> +	if (r) {
> +		/* Fallback through get_user()/put_user(), this may
> +		 * lead false positive that userspace may see more
> +		 * dirty pages. A mutex is used to synchronize log
> +		 * access between vhost threads.
> +		 */
> +		mutex_lock(&vhost_log_lock);
> +		r = get_user(old, addr);
> +		if (r)
> +			goto err;
> +		r = put_user(old | 1 << nr, addr);
> +		if (r)
> +			goto err;
> +		mutex_unlock(&vhost_log_lock);
> +	}

Problem is, we always said it's atomic.

This trick will work if userspace only clears bits
in the log, but won't if it sets bits in the log.
E.g. reusing the log structure for vhost-user
will have exactly this effect.




>  	return 0;
> +err:
> +	mutex_unlock(&vhost_log_lock);
> +	return r;
>  }
>  
>  static int log_write(void __user *log_base,
> @@ -1725,13 +1731,13 @@ static int log_write(void __user *log_base,
>  	if (!write_length)
>  		return 0;
>  	write_length += write_address % VHOST_PAGE_SIZE;
> +	log_base = (void __user *)((u64)log_base & ~0x3ULL);
> +	write_page += ((u64)log_base & 0x3ULL) * 8;
>  	for (;;) {
> -		u64 base = (u64)(unsigned long)log_base;
> -		u64 log = base + write_page / 8;
> -		int bit = write_page % 8;
> -		if ((u64)(unsigned long)log != log)
> -			return -EFAULT;
> -		r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
> +		u32 __user *log = (u32 __user *)log_base + write_page / 32;
> +		int bit = write_page % 32;
> +
> +		r = set_bit_to_user(bit, log);
>  		if (r < 0)
>  			return r;
>  		if (write_length <= VHOST_PAGE_SIZE)
> -- 
> 1.8.3.1
Jason Wang May 15, 2019, 2:55 a.m. UTC | #4
On 2019/5/15 上午6:16, Michael S. Tsirkin wrote:
> On Mon, May 13, 2019 at 01:27:45AM -0400, Jason Wang wrote:
>> Vhost log dirty pages directly to a userspace bitmap through GUP and
>> kmap_atomic() since kernel doesn't have a set_bit_to_user()
>> helper. This will cause issues for the arch that has virtually tagged
>> caches. The way to fix is to keep using userspace virtual
>> address. Fortunately, futex has arch_futex_atomic_op_inuser() which
>> could be used for setting a bit to user.
>>
>> Note there're several cases that futex helper can fail e.g a page
>> fault or the arch that doesn't have the support. For those cases, a
>> simplified get_user()/put_user() pair protected by a global mutex is
>> provided as a fallback. The fallback may lead false positive that
>> userspace may see more dirty pages.
>>
>> Cc: Christoph Hellwig<hch@infradead.org>
>> Cc: James Bottomley<James.Bottomley@HansenPartnership.com>
>> Cc: Andrea Arcangeli<aarcange@redhat.com>
>> Cc: Thomas Gleixner<tglx@linutronix.de>
>> Cc: Ingo Molnar<mingo@redhat.com>
>> Cc: Peter Zijlstra<peterz@infradead.org>
>> Cc: Darren Hart<dvhart@infradead.org>
>> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
>> Signed-off-by: Jason Wang<jasowang@redhat.com>
>> ---
>> Changes from RFC V2:
>> - drop GUP and provide get_user()/put_user() fallbacks
>> - round down log_base
>> Changes from RFC V1:
>> - switch to use arch_futex_atomic_op_inuser()
>> ---
>>   drivers/vhost/vhost.c | 54 ++++++++++++++++++++++++++++-----------------------
>>   1 file changed, 30 insertions(+), 24 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 351af88..7fa05ba 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -31,6 +31,7 @@
>>   #include <linux/sched/signal.h>
>>   #include <linux/interval_tree_generic.h>
>>   #include <linux/nospec.h>
>> +#include <asm/futex.h>
>>   
>>   #include "vhost.h"
>>   
>> @@ -43,6 +44,8 @@
>>   MODULE_PARM_DESC(max_iotlb_entries,
>>   	"Maximum number of iotlb entries. (default: 2048)");
>>   
>> +static DEFINE_MUTEX(vhost_log_lock);
>> +
>>   enum {
>>   	VHOST_MEMORY_F_LOG = 0x1,
>>   };
>> @@ -1692,28 +1695,31 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>>   }
>>   EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
>>   
>> -/* TODO: This is really inefficient.  We need something like get_user()
>> - * (instruction directly accesses the data, with an exception table entry
>> - * returning -EFAULT). See Documentation/x86/exception-tables.txt.
>> - */
>> -static int set_bit_to_user(int nr, void __user *addr)
>> +static int set_bit_to_user(int nr, u32 __user *addr)
>>   {
>> -	unsigned long log = (unsigned long)addr;
>> -	struct page *page;
>> -	void *base;
>> -	int bit = nr + (log % PAGE_SIZE) * 8;
>> +	u32 old;
>>   	int r;
>>   
>> -	r = get_user_pages_fast(log, 1, 1, &page);
>> -	if (r < 0)
>> -		return r;
>> -	BUG_ON(r != 1);
>> -	base = kmap_atomic(page);
>> -	set_bit(bit, base);
>> -	kunmap_atomic(base);
>> -	set_page_dirty_lock(page);
>> -	put_page(page);
>> +	r = arch_futex_atomic_op_inuser(FUTEX_OP_OR, 1 << nr, &old, addr);
>> +	if (r) {
>> +		/* Fallback through get_user()/put_user(), this may
>> +		 * lead false positive that userspace may see more
>> +		 * dirty pages. A mutex is used to synchronize log
>> +		 * access between vhost threads.
>> +		 */
>> +		mutex_lock(&vhost_log_lock);
>> +		r = get_user(old, addr);
>> +		if (r)
>> +			goto err;
>> +		r = put_user(old | 1 << nr, addr);
>> +		if (r)
>> +			goto err;
>> +		mutex_unlock(&vhost_log_lock);
>> +	}
> Problem is, we always said it's atomic.
>
> This trick will work if userspace only clears bits
> in the log, but won't if it sets bits in the log.
> E.g. reusing the log structure for vhost-user
> will have exactly this effect.
>

Ok, I admit this is an issue.

Then I think maybe we can simply fallback to a u8 put_user() which is 
guaranteed to be atomic like:

put_user(0xF, addr);

Then we may at most have 7 more dirty pages to be seen by guest. 
Consider futex helper is implemented is most architectures and it's 
likely to succeed. It should be acceptable.

Does this make sense?

Thanks
diff mbox series

Patch

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 351af88..7fa05ba 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -31,6 +31,7 @@ 
 #include <linux/sched/signal.h>
 #include <linux/interval_tree_generic.h>
 #include <linux/nospec.h>
+#include <asm/futex.h>
 
 #include "vhost.h"
 
@@ -43,6 +44,8 @@ 
 MODULE_PARM_DESC(max_iotlb_entries,
 	"Maximum number of iotlb entries. (default: 2048)");
 
+static DEFINE_MUTEX(vhost_log_lock);
+
 enum {
 	VHOST_MEMORY_F_LOG = 0x1,
 };
@@ -1692,28 +1695,31 @@  long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
 
-/* TODO: This is really inefficient.  We need something like get_user()
- * (instruction directly accesses the data, with an exception table entry
- * returning -EFAULT). See Documentation/x86/exception-tables.txt.
- */
-static int set_bit_to_user(int nr, void __user *addr)
+static int set_bit_to_user(int nr, u32 __user *addr)
 {
-	unsigned long log = (unsigned long)addr;
-	struct page *page;
-	void *base;
-	int bit = nr + (log % PAGE_SIZE) * 8;
+	u32 old;
 	int r;
 
-	r = get_user_pages_fast(log, 1, 1, &page);
-	if (r < 0)
-		return r;
-	BUG_ON(r != 1);
-	base = kmap_atomic(page);
-	set_bit(bit, base);
-	kunmap_atomic(base);
-	set_page_dirty_lock(page);
-	put_page(page);
+	r = arch_futex_atomic_op_inuser(FUTEX_OP_OR, 1 << nr, &old, addr);
+	if (r) {
+		/* Fallback through get_user()/put_user(), this may
+		 * lead false positive that userspace may see more
+		 * dirty pages. A mutex is used to synchronize log
+		 * access between vhost threads.
+		 */
+		mutex_lock(&vhost_log_lock);
+		r = get_user(old, addr);
+		if (r)
+			goto err;
+		r = put_user(old | 1 << nr, addr);
+		if (r)
+			goto err;
+		mutex_unlock(&vhost_log_lock);
+	}
 	return 0;
+err:
+	mutex_unlock(&vhost_log_lock);
+	return r;
 }
 
 static int log_write(void __user *log_base,
@@ -1725,13 +1731,13 @@  static int log_write(void __user *log_base,
 	if (!write_length)
 		return 0;
 	write_length += write_address % VHOST_PAGE_SIZE;
+	log_base = (void __user *)((u64)log_base & ~0x3ULL);
+	write_page += ((u64)log_base & 0x3ULL) * 8;
 	for (;;) {
-		u64 base = (u64)(unsigned long)log_base;
-		u64 log = base + write_page / 8;
-		int bit = write_page % 8;
-		if ((u64)(unsigned long)log != log)
-			return -EFAULT;
-		r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
+		u32 __user *log = (u32 __user *)log_base + write_page / 32;
+		int bit = write_page % 32;
+
+		r = set_bit_to_user(bit, log);
 		if (r < 0)
 			return r;
 		if (write_length <= VHOST_PAGE_SIZE)