Subject: Re: [PATCH v5 07/11] net/9p: limit 'msize' to KMALLOC_MAX_SIZE for all transports

From: Dominique Martinet <[email-masked]>
Date: Wed, 13 Jul 2022 05:38:38 +0900

Christian Schoenebeck wrote on Tue, Jul 12, 2022 at 04:31:26PM +0200:
> This 9p client implementation is yet using linear message buffers for
> most message types, i.e. they use kmalloc() et al. for allocating
> continuous physical memory pages, which is usually limited to 4MB
> buffers. Use KMALLOC_MAX_SIZE though instead of a hard coded 4MB for
> constraining this more safely.
> 
> Unfortunately we cannot simply replace the existing kmalloc() calls by
> vmalloc() ones, because that would yield in non-logical kernel addresses
> (for any vmalloc(>4MB) that is) which are in general not accessible by
> hosts like QEMU.
> 
> In future we would replace those linear buffers by scatter/gather lists
> to eventually get rid of this limit (struct p9_fcall's sdata member by
> p9_fcall_init() and struct p9_fid's rdir member by
> v9fs_alloc_rdir_buf()).
> 
> Signed-off-by: Christian Schoenebeck <[email-masked]>
> ---
> 
> Hmm, that's a bit too simple, as we also need a bit of headroom for
> transport specific overhead. So maybe this has to be handled by each
> transport appropriately instead?

hm yes I'd say it's redundant with each transports max size already --
let's just keep appropriate max values in each transport.

> 
>  net/9p/client.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 20054addd81b..fab939541c81 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -1042,6 +1042,17 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
>  	p9_debug(P9_DEBUG_MUX, "clnt %p trans %p msize %d protocol %d\n",
>  		 clnt, clnt->trans_mod, clnt->msize, clnt->proto_version);
>  
> +	/*
> +	 * due to linear message buffers being used by client ATM
> +	 */
> +	if (clnt->msize > KMALLOC_MAX_SIZE) {
> +		clnt->msize = KMALLOC_MAX_SIZE;
> +		pr_info("Limiting 'msize' to %zu as this is the maximum "
> +			"supported by this client version.\n",
> +			(size_t) KMALLOC_MAX_SIZE
> +		);
> +	}
> +
>  	err = clnt->trans_mod->create(clnt, dev_name, options);
>  	if (err)
>  		goto put_trans;