From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-fsdevel-owner@vger.kernel.org>
Received: from bombadil.infradead.org ([198.137.202.133]:48842 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1725853AbeGaEZC (ORCPT
        <rfc822;linux-fsdevel@vger.kernel.org>);
        Tue, 31 Jul 2018 00:25:02 -0400
Date: Mon, 30 Jul 2018 19:46:58 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Dominique Martinet <asmadeus@codewreck.org>
Cc: v9fs-developer@lists.sourceforge.net, Greg Kurz <groug@kaod.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] net/9p: add a per-client fcall kmem_cache
Message-ID: <20180731024658.GC19692@bombadil.infradead.org>
References: <20180730093101.GA7894@nautica>
 <1532943263-24378-1-git-send-email-asmadeus@codewreck.org>
 <1532943263-24378-2-git-send-email-asmadeus@codewreck.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1532943263-24378-2-git-send-email-asmadeus@codewreck.org>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

On Mon, Jul 30, 2018 at 11:34:23AM +0200, Dominique Martinet wrote:
> -static int p9_fcall_alloc(struct p9_fcall *fc, int alloc_msize)
> +static int p9_fcall_alloc(struct p9_client *c, struct p9_fcall *fc,
> +			  int alloc_msize)
>  {
> -	fc->sdata = kmalloc(alloc_msize, GFP_NOFS);
> +	if (c->fcall_cache && alloc_msize == c->msize)
> +		fc->sdata = kmem_cache_alloc(c->fcall_cache, GFP_NOFS);
> +	else
> +		fc->sdata = kmalloc(alloc_msize, GFP_NOFS);

Could you simplify this by initialising c->msize to 0 and then this
can simply be:

> +	if (alloc_msize == c->msize)
...

> +void p9_fcall_free(struct p9_client *c, struct p9_fcall *fc)
> +{
> +	/* sdata can be NULL for interrupted requests in trans_rdma,
> +	 * and kmem_cache_free does not do NULL-check for us
> +	 */
> +	if (unlikely(!fc->sdata))
> +		return;
> +
> +	if (c->fcall_cache && fc->capacity == c->msize)
> +		kmem_cache_free(c->fcall_cache, fc->sdata);
> +	else
> +		kfree(fc->sdata);
> +}

Is it possible for fcall_cache to be allocated before fcall_free is
called?  I'm concerned we might do this:

allocate message A
allocate message B
receive response A
allocate fcall_cache
receive response B

and then we'd call kmem_cache_free() for something allocated by kmalloc(),
which works with slab and slub, but doesn't work with slob (alas).

> @@ -980,6 +1000,9 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
>  	if (err)
>  		goto close_trans;
>  
> +	clnt->fcall_cache = kmem_cache_create("9p-fcall-cache", clnt->msize,
> +					      0, 0, NULL);
> +

If we have slab merging turned off, or we have two mounts from servers
with different msizes, we'll end up with two slabs called 9p-fcall-cache.
I'm OK with that, but are you?