linux-kernel - Re: [PATCH 1/4] mm: Implement kmem objects freeing queue

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Date:   Fri, 22 Mar 2019 17:47:18 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Waiman Long <longman@...hat.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        selinux@...r.kernel.org, Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1/4] mm: Implement kmem objects freeing queue

On Thu, 21 Mar 2019, Waiman Long wrote:

> When releasing kernel data structures, freeing up the memory
> occupied by those objects is usually the last step. To avoid races,
> the release operation is commonly done with a lock held. However, the
> freeing operations do not need to be under lock, but are in many cases.
>
> In some complex cases where the locks protect many different memory
> objects, that can be a problem especially if some memory debugging
> features like KASAN are enabled. In those cases, freeing memory objects
> under lock can greatly lengthen the lock hold time. This can even lead
> to soft/hard lockups in some extreme cases.
>
> To make it easer to defer freeing memory objects until after unlock,
> a kernel memory freeing queue mechanism is now added. It is modelled
> after the wake_q mechanism for waking up tasks without holding a lock.

It is already pretty easy. You just store the pointer to the slab object
in a local variable, finish all the unlocks and then free the objects.
This is done in numerous places of the kernel.

I fear that the automated mechanism will make the code more difficult to
read and result in a loss of clarity of the sequencing of events in
releasing locks and objects.

Also there is already kfree_rcu which does a similar thing to what you are
proposing here and is used in numerous places.