linux-kernel - Re: [PATCH 01/14] wireguard: allowedips: replace call_rcu by kfree_rcu for simple kmem_cache

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Message-ID: <3f58c9a6-614f-4188-9a38-72c26fb42c8e@suse.cz>
Date: Mon, 10 Jun 2024 22:38:08 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: "Jason A. Donenfeld" <Jason@...c4.com>,
 Julia Lawall <Julia.Lawall@...ia.fr>
Cc: kernel-janitors@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, wireguard@...ts.zx2c4.com,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 "Paul E . McKenney" <paulmck@...nel.org>,
 Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 01/14] wireguard: allowedips: replace call_rcu by
 kfree_rcu for simple kmem_cache_free callback

On 6/9/24 4:32 PM, Jason A. Donenfeld wrote:
> Hi Julia & Vlastimil,
> 
> On Sun, Jun 09, 2024 at 10:27:13AM +0200, Julia Lawall wrote:
>> Since SLOB was removed, it is not necessary to use call_rcu
>> when the callback only performs kmem_cache_free. Use
>> kfree_rcu() directly.
> 
> Thanks, I applied this to the wireguard tree, and I'll send this out as
> a fix for 6.10. Let me know if this is unfavorable to you and if you'd
> like to take this somewhere yourself, in which case I'll give you my
> ack.
> 
> Just a question, though, for Vlastimil -- I know that with the SLOB
> removal, kfree() is now allowed on kmemcache'd objects. Do you plan to
> do a blanket s/kmem_cache_free/kfree/g at some point, and then remove
> kmem_cache_free all together?

Hmm, not really, but obligatory Cc for willy who'd love to have "one free()
to rule them all" IIRC.

My current thinking is that kmem_cache_free() can save the kmem_cache
lookup, or serve as a double check if debugging is enabled, and doesn't have
much downside. If someone wants to not care about the kmem_cache pointer,
they can use kfree(). Even convert their subsystem at will. But a mass
conversion of everything would be rather lot of churn for not much of a
benefit, IMHO.