Re: [tipc-discussion] [PATCH v4 net-next 1/2] net: tipc: fix FB_MTU eat two pages

[Jon M. <jm...@re...>]

On 6/18/21 2:57 AM, men...@gm... wrote:
> From: Menglong Dong <don...@zt...>
>
> FB_MTU is used in 'tipc_msg_build()' to alloc smaller skb when memory
> allocation fails, which can avoid unnecessary sending failures.
>
> The value of FB_MTU now is 3744, and the data size will be:
>
>    (3744 + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) + \
>      SKB_DATA_ALIGN(BUF_HEADROOM + BUF_TAILROOM + 3))
>
> which is larger than one page(4096), and two pages will be allocated.
>
> To avoid it, replace '3744' with a calculation:
>
>    (PAGE_SIZE - SKB_DATA_ALIGN(BUF_OVERHEAD) - \
>      SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))
>
> What's more, alloc_skb_fclone() will call SKB_DATA_ALIGN for data size,
> and it's not necessary to make alignment for buf_size in
> tipc_buf_acquire(). So, just remove it.
>
> Fixes: 4c94cc2d3d57 ("tipc: fall back to smaller MTU if allocation of local send skb fails")
>
> Signed-off-by: Menglong Dong <don...@zt...>
> ---
> V4:
> - fallback to V2
>
> V3:
> - split tipc_msg_build to tipc_msg_build and tipc_msg_frag
> - introduce tipc_buf_acquire_flex, which is able to alloc skb for
>    local message
> - add the variate 'local' in tipc_msg_build to check if this is a
>    local message.
>
> V2:
> - define FB_MTU in msg.c instead of introduce a new file
> - remove align for buf_size in tipc_buf_acquire()
> ---
>   net/tipc/bcast.c |  2 +-
>   net/tipc/msg.c   | 19 ++++++++++---------
>   net/tipc/msg.h   |  3 ++-
>   3 files changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/net/tipc/bcast.c b/net/tipc/bcast.c
> index d4beca895992..593846d25214 100644
> --- a/net/tipc/bcast.c
> +++ b/net/tipc/bcast.c
> @@ -699,7 +699,7 @@ int tipc_bcast_init(struct net *net)
>   	spin_lock_init(&tipc_net(net)->bclock);
>   
>   	if (!tipc_link_bc_create(net, 0, 0, NULL,
> -				 FB_MTU,
> +				 one_page_mtu,
>   				 BCLINK_WIN_DEFAULT,
>   				 BCLINK_WIN_DEFAULT,
>   				 0,
> diff --git a/net/tipc/msg.c b/net/tipc/msg.c
> index ce6ab54822d8..912d17b3fc01 100644
> --- a/net/tipc/msg.c
> +++ b/net/tipc/msg.c
> @@ -44,12 +44,17 @@
>   #define MAX_FORWARD_SIZE 1024
>   #ifdef CONFIG_TIPC_CRYPTO
>   #define BUF_HEADROOM ALIGN(((LL_MAX_HEADER + 48) + EHDR_MAX_SIZE), 16)
> -#define BUF_TAILROOM (TIPC_AES_GCM_TAG_SIZE)
> +#define BUF_OVERHEAD (BUF_HEADROOM + TIPC_AES_GCM_TAG_SIZE)
>   #else
>   #define BUF_HEADROOM (LL_MAX_HEADER + 48)
> -#define BUF_TAILROOM 16
> +#define BUF_OVERHEAD BUF_HEADROOM
>   #endif
>   
> +#define ONE_PAGE_SKB_SZ (PAGE_SIZE - SKB_DATA_ALIGN(BUF_OVERHEAD) - \
> +			 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))
> +
I suggest one small simplification:
Let's just skip this macro completely, and assign the calculation 
directly to one_page_mtu below.

Otherwise:
Acked-by: Jon Maloy <jm...@re...>

> +const int one_page_mtu = ONE_PAGE_SKB_SZ;
> +
>   static unsigned int align(unsigned int i)
>   {
>   	return (i + 3) & ~3u;
> @@ -69,13 +74,8 @@ static unsigned int align(unsigned int i)
>   struct sk_buff *tipc_buf_acquire(u32 size, gfp_t gfp)
>   {
>   	struct sk_buff *skb;
> -#ifdef CONFIG_TIPC_CRYPTO
> -	unsigned int buf_size = (BUF_HEADROOM + size + BUF_TAILROOM + 3) & ~3u;
> -#else
> -	unsigned int buf_size = (BUF_HEADROOM + size + 3) & ~3u;
> -#endif
>   
> -	skb = alloc_skb_fclone(buf_size, gfp);
> +	skb = alloc_skb_fclone(BUF_OVERHEAD + size, gfp);
>   	if (skb) {
>   		skb_reserve(skb, BUF_HEADROOM);
>   		skb_put(skb, size);
> @@ -395,7 +395,8 @@ int tipc_msg_build(struct tipc_msg *mhdr, struct msghdr *m, int offset,
>   		if (unlikely(!skb)) {
>   			if (pktmax != MAX_MSG_SIZE)
>   				return -ENOMEM;
> -			rc = tipc_msg_build(mhdr, m, offset, dsz, FB_MTU, list);
> +			rc = tipc_msg_build(mhdr, m, offset, dsz,
> +					    ONE_PAGE_SKB_SZ, list);
>   			if (rc != dsz)
>   				return rc;
>   			if (tipc_msg_assemble(list))
> diff --git a/net/tipc/msg.h b/net/tipc/msg.h
> index 5d64596ba987..64ae4c4c44f8 100644
> --- a/net/tipc/msg.h
> +++ b/net/tipc/msg.h
> @@ -99,9 +99,10 @@ struct plist;
>   #define MAX_H_SIZE                60	/* Largest possible TIPC header size */
>   
>   #define MAX_MSG_SIZE (MAX_H_SIZE + TIPC_MAX_USER_MSG_SIZE)
> -#define FB_MTU                  3744
>   #define TIPC_MEDIA_INFO_OFFSET	5
>   
> +extern const int one_page_mtu;
> +
>   struct tipc_skb_cb {
>   	union {
>   		struct {