Re: [PATCH v2] 9p: Use kvmalloc for message buffers on supported transports

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Thursday, October 16, 2025 9:01:56 AM CEST Pierre Barre wrote:
> While developing a 9P server (https://github.com/Barre/ZeroFS) and
> testing it under high-load, I was running into allocation failures.
> The failures occur even with plenty of free memory available because
> kmalloc requires contiguous physical memory.
> 
> This results in errors like:
> ls: page allocation failure: order:7, mode:0x40c40(GFP_NOFS|__GFP_COMP)
> 
> This patch introduces a transport capability flag (supports_vmalloc)
> that indicates whether a transport can work with vmalloc'd buffers
> (non-physically contiguous memory). Transports requiring DMA should
> leave this flag as false.
> 
> The fd-based transports (tcp, unix, fd) set this flag to true, and
> p9_fcall_init will use kvmalloc instead of kmalloc for these
> transports. This allows the allocator to fall back to vmalloc when
> contiguous physical memory is not available.
> 
> Additionally, if kmem_cache_alloc fails, the code falls back to
> kvmalloc for transports that support it.
> 
> Signed-off-by: Pierre Barre <pierre@barre.sh>
> ---
> 
>  include/net/9p/transport.h |  4 ++++
>  net/9p/client.c            | 11 +++++++++--
>  net/9p/trans_fd.c          |  3 +++
>  3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/9p/transport.h b/include/net/9p/transport.h
> index 766ec07c9599..f0981515148d 100644
> --- a/include/net/9p/transport.h
> +++ b/include/net/9p/transport.h
> @@ -24,6 +24,9 @@
>   *                   we're less flexible when choosing the response message
> *                   size in this case
>   * @def: set if this transport should be considered the default
> + * @supports_vmalloc: set if this transport can work with vmalloc'd buffers
> + *                    (non-physically contiguous memory). Transports
> requiring + *                    DMA should leave this as false.
>   * @create: member function to create a new connection on this transport
>   * @close: member function to discard a connection on this transport
>   * @request: member function to issue a request to the transport
> @@ -44,6 +47,7 @@ struct p9_trans_module {
>  	int maxsize;		/* max message size of transport */
>  	bool pooled_rbuffers;
>  	int def;		/* this transport should be default */
> +	bool supports_vmalloc;	/* can work with vmalloc'd buffers */
>  	struct module *owner;
>  	int (*create)(struct p9_client *client,
>  		      const char *devname, char *args);
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 5c1ca57ccd28..2a4884c880c1 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -229,8 +229,15 @@ static int p9_fcall_init(struct p9_client *c, struct
> p9_fcall *fc, if (likely(c->fcall_cache) && alloc_msize == c->msize) {
>  		fc->sdata = kmem_cache_alloc(c->fcall_cache, GFP_NOFS);
>  		fc->cache = c->fcall_cache;
> +		if (!fc->sdata && c->trans_mod->supports_vmalloc) {
> +			fc->sdata = kvmalloc(alloc_msize, GFP_NOFS);
> +			fc->cache = NULL;
> +		}
>  	} else {
> -		fc->sdata = kmalloc(alloc_msize, GFP_NOFS);
> +		if (c->trans_mod->supports_vmalloc)
> +			fc->sdata = kvmalloc(alloc_msize, GFP_NOFS);
> +		else
> +			fc->sdata = kmalloc(alloc_msize, GFP_NOFS);
>  		fc->cache = NULL;
>  	}
>  	if (!fc->sdata)
> @@ -252,7 +259,7 @@ void p9_fcall_fini(struct p9_fcall *fc)
>  	if (fc->cache)
>  		kmem_cache_free(fc->cache, fc->sdata);
>  	else
> -		kfree(fc->sdata);
> +		kvfree(fc->sdata);
>  }
>  EXPORT_SYMBOL(p9_fcall_fini);
> 
> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
> index a516745f732f..e7334033eba5 100644
> --- a/net/9p/trans_fd.c
> +++ b/net/9p/trans_fd.c
> @@ -1101,6 +1101,7 @@ static struct p9_trans_module p9_tcp_trans = {
>  	.maxsize = MAX_SOCK_BUF,
>  	.pooled_rbuffers = false,
>  	.def = 0,
> +	.supports_vmalloc = true,
>  	.create = p9_fd_create_tcp,
>  	.close = p9_fd_close,n
>  	.request = p9_fd_request,
> @@ -1115,6 +1116,7 @@ static struct p9_trans_module p9_unix_trans = {
>  	.name = "unix",
>  	.maxsize = MAX_SOCK_BUF,
>  	.def = 0,
> +	.supports_vmalloc = true,
>  	.create = p9_fd_create_unix,
>  	.close = p9_fd_close,
>  	.request = p9_fd_request,
> @@ -1129,6 +1131,7 @@ static struct p9_trans_module p9_fd_trans = {
>  	.name = "fd",
>  	.maxsize = MAX_SOCK_BUF,
>  	.def = 0,
> +	.supports_vmalloc = true,
>  	.create = p9_fd_create,
>  	.close = p9_fd_close,
>  	.request = p9_fd_request,

Just for clarity I would add .supports_vmalloc = false to the other 
transports. Except of that:

Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>

Thanks!

/Christian