Subject: Re: [PATCH net-next v2] net/smc: cap allocation order for SMC-R physically contiguous buffers

From: Simon Horman <[email-masked]>
Date: Fri, 10 Apr 2026 16:16:31 +0100

On Tue, Apr 07, 2026 at 08:43:37PM +0800, D. Wythe wrote:
> The alloc_pages() cannot satisfy requests exceeding MAX_PAGE_ORDER,
> and attempting such allocations will lead to guaranteed failures
> and potential kernel warnings.
> 
> For SMCR_PHYS_CONT_BUFS, cap the allocation order to MAX_PAGE_ORDER.
> This ensures the attempts to allocate the largest possible physically
> contiguous chunk succeed, instead of failing with an invalid order.
> This also avoids redundant "try-fail-degrade" cycles in
> __smc_buf_create().
> 
> For SMCR_MIXED_BUFS, no cap is needed: if the order exceeds
> MAX_PAGE_ORDER, alloc_pages() will silently fail (__GFP_NOWARN)
> and automatically fall back to virtual memory.
> 
> Signed-off-by: D. Wythe <[email-masked]>
> Reviewed-by: Dust Li <[email-masked]>
> ---
> Changes v1 -> v2:
> https://lore.kernel.org/netdev/[email-masked]/
> 
> - Move the bufsize cap from smcr_new_buf_create() up to
>   __smc_buf_create(), which is simpler and avoids touching
>   the allocation logic itself.

The nit below notwithstanding, this looks good to me.

Reviewed-by: Simon Horman <[email-masked]>

> ---
>  net/smc/smc_core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
> index e2d083daeb7e..cdd881746e21 100644
> --- a/net/smc/smc_core.c
> +++ b/net/smc/smc_core.c
> @@ -2440,6 +2440,10 @@ static int __smc_buf_create(struct smc_sock *smc, bool is_smcd, bool is_rmb)
>  		/* use socket send buffer size (w/o overhead) as start value */
>  		bufsize = smc->sk.sk_sndbuf / 2;
>  
> +	/* limit bufsize for physically contiguous buffers */
> +	if (!is_smcd && lgr->buf_type == SMCR_PHYS_CONT_BUFS)
> +		bufsize = min_t(int, bufsize, (PAGE_SIZE << MAX_PAGE_ORDER));

nit: I think min() is sufficient here, and the inner parentheses are
     unnecessary

> +
>  	for (bufsize_comp = smc_compress_bufsize(bufsize, is_smcd, is_rmb);
>  	     bufsize_comp >= 0; bufsize_comp--) {
>  		if (is_rmb) {
> -- 
> 2.45.0
>