Linux Virtualization - Nov 2010 - [PATCH] vhost: correctly set bits of dirty pages

When counting pages we should increase it by 1 instead of VHOST_PAGE_SIZE,
and also make log_write() can correctly process the request across
pages with write_address not start at page boundary.

Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/vhost/vhost.c |   20 +++++++++-----------
 1 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index a29d91c..576300b 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -884,23 +884,21 @@ static int set_bit_to_user(int nr, void __user *addr)
 static int log_write(void __user *log_base,
 		     u64 write_address, u64 write_length)
 {
-	int r;
-	if (!write_length)
-		return 0;
-	write_address /= VHOST_PAGE_SIZE;
-	for (;;) {
+	int r = 0;
+	while (write_length > 0) {
+		u64 l = VHOST_PAGE_SIZE - write_address % VHOST_PAGE_SIZE;
+		u64 write_page = write_address / VHOST_PAGE_SIZE;
 		u64 base = (u64)(unsigned long)log_base;
-		u64 log = base + write_address / 8;
-		int bit = write_address % 8;
+		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);
 		if (r < 0)
 			return r;
-		if (write_length <= VHOST_PAGE_SIZE)
-			break;
-		write_length -= VHOST_PAGE_SIZE;
-		write_address += VHOST_PAGE_SIZE;
+		l = min(l, write_length);
+		write_length -= l;
+		write_address += l;
 	}
 	return r;
 }

On Mon, Nov 29, 2010 at 01:48:20PM +0800, Jason Wang
wrote:
> When counting pages we should increase it by 1 instead of VHOST_PAGE_SIZE,
> and also make log_write() can correctly process the request across
> pages with write_address not start at page boundary.
> 
> Signed-off-by: Jason Wang <jasowang at redhat.com>

Thanks, good catch!
But let's to it in small steps: first, a small patch to fix the bug:
I think this is equivalent, right?

Subject: vhost: correctly set bits of dirty pages

When counting pages we should increase address by 1 instead of
VHOST_PAGE_SIZE, and also make log_write() can correctly process the
request across pages with write_address not starting at page boundary.

Reported-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>

---

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 568eb70..d0a3552 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -887,6 +887,7 @@ static int log_write(void __user *log_base,
 	int r;
 	if (!write_length)
 		return 0;
+	write_length += write_address % VHOST_PAGE_SIZE;
 	write_address /= VHOST_PAGE_SIZE;
 	for (;;) {
 		u64 base = (u64)(unsigned long)log_base;
@@ -900,7 +901,7 @@ static int log_write(void __user *log_base,
 		if (write_length <= VHOST_PAGE_SIZE)
 			break;
 		write_length -= VHOST_PAGE_SIZE;
-		write_address += VHOST_PAGE_SIZE;
+		write_address += 1;
 	}
 	return r;
 }