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

[Manfred Spraul]

Hi Andrew,

On 1/27/22 03:53, Andrew Morton wrote:
> On Wed, 22 Dec 2021 20:48:28 +0100 Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> 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);
> >   }