linux-kernel - Re: [PATCH] 9p: Use kvmalloc for message buffers

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Message-ID: <2989343.ydHz1Oe0dO@silver>
Date: Wed, 30 Jul 2025 18:08:13 +0200
From: Christian Schoenebeck <linux_oss@...debyte.com>
To: v9fs@...ts.linux.dev, Pierre Barre <pierre@...re.sh>
Cc: ericvh@...nel.org, lucho@...kov.net, asmadeus@...ewreck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] 9p: Use kvmalloc for message buffers

On Wednesday, July 30, 2025 5:08:05 PM 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)

What was msize?

> Signed-off-by: Pierre Barre <pierre@...re.sh>
> ---
>  net/9p/client.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 5c1ca57ccd28..f82b5674057c 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -230,7 +230,7 @@ static int p9_fcall_init(struct p9_client *c, struct p9_fcall *fc,
>  		fc->sdata = kmem_cache_alloc(c->fcall_cache, GFP_NOFS);
>  		fc->cache = c->fcall_cache;
>  	} else {
> -		fc->sdata = kmalloc(alloc_msize, GFP_NOFS);
> +		fc->sdata = kvmalloc(alloc_msize, GFP_NOFS);

That would work with certain transports like fd I guess, but not via
virtio-pci transport for instance, since PCI-DMA requires physical pages. Same
applies to Xen transport I guess.

>  		fc->cache = NULL;
>  	}
>  	if (!fc->sdata)
> @@ -252,7 +252,7 @@ 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);
>  
>