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

[D. Wythe]

On Fri, Apr 10, 2026 at 04:16:31PM +0100, Simon Horman wrote:
> 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 <alibuda@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Dust Li <dust.li@xxxxxxxxxxxxxxxxx>
> > ---
> > Changes v1 -> v2:
> > https://lore.kernel.org/netdev/20260312082154.36971-1-alibuda@xxxxxxxxxxxxxxxxx/
> > 
> > - 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 <horms@xxxxxxxxxx>
> 
> > ---
> >  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

Hi Simon,

I think min_t is required here because min() triggers a signedness
error:

././include/linux/compiler_types.h:706:38: error: call to
‘__compiletime_assert_950’ declared with attribute error: min(bufsize,
((1UL) << 12) << 10) signedness error

The inner parentheses can be removed, though.

D. Wythe

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