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

From: Jakub Kicinski <[email-masked]>
Date: Mon, 16 Mar 2026 19:45:34 -0700

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---
net/smc: cap allocation order for SMC-R physically contiguous buffers

The alloc_page() 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. For SMCR_MIXED_BUFS, if its order
exceeds MAX_PAGE_ORDER, skip the physical allocation attempt and fallback to
virtual memory immediately.

> diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
> index e2d083daeb7e..a18730edb7e0 100644
> --- a/net/smc/smc_core.c
> +++ b/net/smc/smc_core.c
> @@ -2314,6 +2314,10 @@ int smcr_buf_reg_lgr(struct smc_link *lnk)
>  	return rc;
>  }
>
> +/*
> + * smcr_new_buf_create may allocate a buffer smaller than the requested
> + * bufsize. Use buf_desc->len to determine the actual allocated size.
> + */
>  static struct smc_buf_desc *smcr_new_buf_create(struct smc_link_group *lgr,
>  						int bufsize)
>  {
> @@ -2326,18 +2330,22 @@ static struct smc_buf_desc *smcr_new_buf_create(struct smc_link_group *lgr,
>
>  	switch (lgr->buf_type) {
>  	case SMCR_PHYS_CONT_BUFS:
> +		bufsize = min(bufsize, (int)PAGE_SIZE << MAX_PAGE_ORDER);
> +		fallthrough;
>  	case SMCR_MIXED_BUFS:
>  		buf_desc->order = get_order(bufsize);
> -		buf_desc->pages = alloc_pages(GFP_KERNEL | __GFP_NOWARN |
> -					      __GFP_NOMEMALLOC | __GFP_COMP |
> -					      __GFP_NORETRY | __GFP_ZERO,
> -					      buf_desc->order);
> -		if (buf_desc->pages) {
> -			buf_desc->cpu_addr =
> -				(void *)page_address(buf_desc->pages);
> -			buf_desc->len = bufsize;
> -			buf_desc->is_vm = false;
> -			break;
> +		if (buf_desc->order <= MAX_PAGE_ORDER) {
> +			buf_desc->pages = alloc_pages(GFP_KERNEL | __GFP_NOWARN |
> +						      __GFP_NOMEMALLOC | __GFP_COMP |
> +						      __GFP_NORETRY | __GFP_ZERO,
> +						      buf_desc->order);
> +			if (buf_desc->pages) {
> +				buf_desc->cpu_addr =
> +					(void *)page_address(buf_desc->pages);
> +				buf_desc->len = bufsize;
> +				buf_desc->is_vm = false;
> +				break;
> +			}
>  		}
>  		if (lgr->buf_type == SMCR_PHYS_CONT_BUFS)
>  			goto out;

[ ... ]

> @@ -2476,7 +2484,7 @@ static int __smc_buf_create(struct smc_sock *smc, bool is_smcd, bool is_rmb)
>  		}
>
>  		SMC_STAT_RMB_ALLOC(smc, is_smcd, is_rmb);
> -		SMC_STAT_RMB_SIZE(smc, is_smcd, is_rmb, true, bufsize);
> +		SMC_STAT_RMB_SIZE(smc, is_smcd, is_rmb, true, buf_desc->len);
>  		buf_desc->used = 1;
>  		down_write(lock);
>  		smc_lgr_buf_list_add(lgr, is_rmb, buf_list, buf_desc);
>  		up_write(lock);
>  		break; /* found */
>  	}
>
>  	if (IS_ERR(buf_desc))
>  		return PTR_ERR(buf_desc);
>
>  	if (!is_smcd) {
>  		if (smcr_buf_map_usable_links(lgr, buf_desc, is_rmb)) {
>  			smcr_buf_unuse(buf_desc, is_rmb, lgr);
>  			return -ENOMEM;
>  		}
>  	}
>
>  	if (is_rmb) {
>  		conn->rmb_desc = buf_desc;
>  		conn->rmbe_size_comp = bufsize_comp;
>  		smc->sk.sk_rcvbuf = bufsize * 2;
>  		atomic_set(&conn->bytes_to_rcv, 0);
>  		conn->rmbe_update_limit =
>  			smc_rmb_wnd_update_limit(buf_desc->len);
>  		if (is_smcd)
>  			smc_ism_set_conn(conn); /* map RMB/smcd_dev to conn */
>  	} else {
>  		conn->sndbuf_desc = buf_desc;
>  		smc->sk.sk_sndbuf = bufsize * 2;
>  		atomic_set(&conn->sndbuf_space, bufsize);
                                                 ^^^^^^^

The comment added at smcr_new_buf_create() says "Use buf_desc->len to
determine the actual allocated size," but __smc_buf_create() still uses the
uncapped bufsize for several critical fields. Is this correct?

For SMCR_PHYS_CONT_BUFS, smcr_new_buf_create() now caps bufsize internally
to PAGE_SIZE << MAX_PAGE_ORDER (typically 4MB), so buf_desc->len can be
smaller than the requested bufsize.

However, __smc_buf_create() continues using the original uncapped bufsize
for:

    if (is_rmb) {
        conn->rmbe_size_comp = bufsize_comp;
        smc->sk.sk_rcvbuf = bufsize * 2;
    } else {
        smc->sk.sk_sndbuf = bufsize * 2;
        atomic_set(&conn->sndbuf_space, bufsize);
    }

The sndbuf_space mismatch is particularly concerning. Looking at
smc_tx.c:274, the comment states: "0 <= sndbuf_space <= sndbuf_desc->len".
When sndbuf_space exceeds sndbuf_desc->len, smc_tx_sendmsg() could calculate
writespace incorrectly:

    int writespace = atomic_read(&conn->sndbuf_space);

and the two-chunk copy loop might write beyond the actual buffer boundary.
This is reachable with default sysctl settings (SMCR_PHYS_CONT_BUFS) and
socket buffer sizes exceeding 4MB.

Should these assignments use buf_desc->len instead of bufsize? For example:

    atomic_set(&conn->sndbuf_space, buf_desc->len);
    smc->sk.sk_sndbuf = buf_desc->len * 2;
    smc->sk.sk_rcvbuf = buf_desc->len * 2;

And should rmbe_size_comp be recalculated from buf_desc->len? The
rmbe_size_comp value is sent to the peer via CLC messages in smc_clc.c:1125,
and an inflated value might cause the peer to believe the RMB is larger than
it actually is.

>  	}
>  	return 0;
>  }
-- 
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