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SubjectRe: [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@colorfullife.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

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