[PATCH] net: core: Modify alloc_size in alloc_netdev_mqs()

Use ALIGN for 'struct net_device', and remove the unneeded
'NETDEV_ALIGN - 1'. This can save a few bytes. and modify
the pr_err content when txqs < 1.

Signed-off-by: Yajun Deng <redacted>
---
 net/core/dev.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index c253c2aafe97..c42a682a624d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -10789,7 +10789,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 	BUG_ON(strlen(name) >= sizeof(dev->name));
 
 	if (txqs < 1) {
-		pr_err("alloc_netdev: Unable to allocate device with zero queues\n");
+		pr_err("alloc_netdev: Unable to allocate device with zero TX queues\n");
 		return NULL;
 	}
 

@@ -10798,14 +10798,12 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 		return NULL;
 	}
 
-	alloc_size = sizeof(struct net_device);
+	/* ensure 32-byte alignment of struct net_device*/
+	alloc_size = ALIGN(sizeof(struct net_device), NETDEV_ALIGN);
 	if (sizeof_priv) {
 		/* ensure 32-byte alignment of private area */
-		alloc_size = ALIGN(alloc_size, NETDEV_ALIGN);
-		alloc_size += sizeof_priv;
+		alloc_size += ALIGN(sizeof_priv, NETDEV_ALIGN);
 	}
-	/* ensure 32-byte alignment of whole construct */
-	alloc_size += NETDEV_ALIGN - 1;
 
 	p = kvzalloc(alloc_size, GFP_KERNEL | __GFP_RETRY_MAYFAIL);
 	if (!p)

-- 
2.32.0

On Wed, Jun 30, 2021 at 7:11 AM Yajun Deng [off-list ref] wrote:

Use ALIGN for 'struct net_device', and remove the unneeded
'NETDEV_ALIGN - 1'. This can save a few bytes. and modify
the pr_err content when txqs < 1.

I think that in old times (maybe still today), SLAB debugging could
lead to not unaligned allocated zones.
The forced alignment for netdev structures came in
commit f346af6a27c0cea99522213cb813fd30489136e2 ("net_device and
netdev private struct allocation improvements.")
in linux-2.6.3 (back in 2004)

This supposedly was a win in itself, otherwise Al Viro would not have
spent time on this.

This is not really needed, because struct net_device is cache line
aligned already on SMP builds.

        if (sizeof_priv) {
                /* ensure 32-byte alignment of private area */
-               alloc_size = ALIGN(alloc_size, NETDEV_ALIGN);
-               alloc_size += sizeof_priv;
+               alloc_size += ALIGN(sizeof_priv, NETDEV_ALIGN);

No longer needed, the private area starts at the end of struct net_device, whose
size is a multiple of cache line.

Really I doubt this makes sense anymore these days, we have hundreds
of structures in the kernel
that would need a similar handling if SLAB/SLUB was doing silly things.

I would simply do :

alloc_size += sizeof_priv;

        }
-       /* ensure 32-byte alignment of whole construct */
-       alloc_size += NETDEV_ALIGN - 1;

        p = kvzalloc(alloc_size, GFP_KERNEL | __GFP_RETRY_MAYFAIL);
        if (!p)
--
2.32.0