linux-kernel - Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding spinlocks

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Date:   Thu, 27 Jan 2022 06:59:50 +0100
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Vasily Averin <vvs@...tuozzo.com>, cgel.zte@...il.com,
        shakeelb@...gle.com, rdunlap@...radead.org, dbueso@...e.de,
        unixbhaskar@...il.com, chi.minghao@....com.cn, arnd@...db.de,
        Zeal Robot <zealci@....com.cn>, linux-mm@...ck.org,
        1vier1@....de, stable@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding
 spinlocks

Hi Andrew,

On 1/27/22 03:53, Andrew Morton wrote:
> On Wed, 22 Dec 2021 20:48:28 +0100 Manfred Spraul <manfred@...orfullife.com> wrote:
>
>> One codepath in find_alloc_undo() calls kvfree() while holding a spinlock.
>> Since vfree() can sleep this is a bug.
>>
>> Previously, the code path used kfree(), and kfree() is safe to be called
>> while holding a spinlock.
>>
>> Minghao proposed to fix this by updating find_alloc_undo().
>>
>> Alternate proposal to fix this: Instead of changing find_alloc_undo(),
>> change kvfree() so that the same rules as for kfree() apply:
>> Having different rules for kfree() and kvfree() just asks for bugs.
>>
>> Disadvantage: Releasing vmalloc'ed memory will be delayed a bit.
> I know we've been around this loop a bunch of times and deferring was
> considered.   But I forget the conclusion.  IIRC, mhocko was involved?

I do not remember a mail from mhocko.

Shakeel proposed to use the approach from Chi.

Decision: https://marc.info/?l=linux-kernel&m=164132032717757&w=2

With Reviewed-by:

https://marc.info/?l=linux-kernel&m=164132744522325&w=2
>> --- a/mm/util.c
>> +++ b/mm/util.c
>> @@ -610,12 +610,12 @@ EXPORT_SYMBOL(kvmalloc_node);
>>    * It is slightly more efficient to use kfree() or vfree() if you are certain
>>    * that you know which one to use.
>>    *
>> - * Context: Either preemptible task context or not-NMI interrupt.
>> + * Context: Any context except NMI interrupt.
>>    */
>>   void kvfree(const void *addr)
>>   {
>>   	if (is_vmalloc_addr(addr))
>> -		vfree(addr);
>> +		vfree_atomic(addr);
>>   	else
>>   		kfree(addr);
>>   }