linux-kernel - [PATCH v3 2/2] Drivers: hv: Make the sysfs node size for the ring buffer dynamic

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Message-Id: <20250328052745.1417-3-namjain@linux.microsoft.com>
Date: Fri, 28 Mar 2025 10:57:45 +0530
From: Naman Jain <namjain@...ux.microsoft.com>
To: "K . Y . Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>,
	Dexuan Cui <decui@...rosoft.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Stephen Hemminger <stephen@...workplumber.org>
Cc: linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	stable@...nel.org,
	Saurabh Sengar <ssengar@...ux.microsoft.com>,
	Michael Kelley <mhklinux@...look.com>,
	Naman Jain <namjain@...ux.microsoft.com>
Subject: [PATCH v3 2/2] Drivers: hv: Make the sysfs node size for the ring buffer dynamic

The ring buffer size varies across VMBus channels. The size of sysfs
node for the ring buffer is currently hardcoded to 4 MB. Userspace
clients either use fstat() or hardcode this size for doing mmap().
To address this, make the sysfs node size dynamic to reflect the
actual ring buffer size for each channel. This will ensure that
fstat() on ring sysfs node always return the correct size of
ring buffer.

Signed-off-by: Naman Jain <namjain@...ux.microsoft.com>
---
 drivers/hv/vmbus_drv.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 66f1c21050d4..c7be85606031 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1820,7 +1820,6 @@ static struct bin_attribute chan_attr_ring_buffer = {
 		.name = "ring",
 		.mode = 0600,
 	},
-	.size = 2 * SZ_2M,
 	.mmap = hv_mmap_ring_buffer_wrapper,
 };
 static struct attribute *vmbus_chan_attrs[] = {
@@ -1880,11 +1879,21 @@ static umode_t vmbus_chan_bin_attr_is_visible(struct kobject *kobj,
 	return attr->attr.mode;
 }
 
+static size_t vmbus_chan_bin_size(struct kobject *kobj,
+				  const struct bin_attribute *bin_attr, int a)
+{
+	const struct vmbus_channel *channel =
+		container_of(kobj, struct vmbus_channel, kobj);
+
+	return channel->ringbuffer_pagecount << PAGE_SHIFT;
+}
+
 static const struct attribute_group vmbus_chan_group = {
 	.attrs = vmbus_chan_attrs,
 	.bin_attrs = vmbus_chan_bin_attrs,
 	.is_visible = vmbus_chan_attr_is_visible,
 	.is_bin_visible = vmbus_chan_bin_attr_is_visible,
+	.bin_size = vmbus_chan_bin_size,
 };
 
 static const struct kobj_type vmbus_chan_ktype = {
-- 
2.34.1