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

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Date:   Thu, 27 Jan 2022 09:25:48 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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
Subject: Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding
 spinlocks

On Thu 27-01-22 06:59:50, Manfred Spraul wrote:
> 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.

I do not remember either.

> 
> Shakeel proposed to use the approach from Chi.
> 
> Decision: https://marc.info/?l=linux-kernel&m=164132032717757&w=2

And I would agree with Shakeel and go with the original change to the
ipc code. That is trivial and without any other side effects like this
one. I bet nobody has evaluated what the undconditional deferred freeing
has. At least changelog doesn't really dive into that more than a very
vague statement that this will happen.

> 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);
> > >   }
> 

-- 
Michal Hocko
SUSE Labs