Re: [PATCH] mm: check for sleepable context in kvfree

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Tue, Jul 23, 2019 at 02:19:03PM -0400, Jeff Layton wrote:
> On Tue, 2019-07-23 at 11:11 -0700, Matthew Wilcox wrote:
> > On Tue, Jul 23, 2019 at 02:05:11PM -0400, Jeff Layton wrote:
> > > On Tue, 2019-07-23 at 10:55 -0700, Matthew Wilcox wrote:
> > > > I think it's a bit of a landmine, to be honest.  How about we have kvfree()
> > > > call vfree_atomic() instead?
> > > 
> > > Not a bad idea, though it means more overhead for the vfree case.
> > > 
> > > Since we're spitballing here...could we have kvfree figure out whether
> > > it's running in a context where it would need to queue it instead and
> > > only do it in that case?
> > > 
> > > We currently have to figure that out for the might_sleep_if anyway. We
> > > could just have it DTRT instead of printk'ing and dumping the stack in
> > > that case.
> > 
> > I don't think we have a generic way to determine if we're currently
> > holding a spinlock.  ie this can fail:
> > 
> > spin_lock(&my_lock);
> > kvfree(p);
> > spin_unlock(&my_lock);
> > 
> > If we're preemptible, we can check the preempt count, but !CONFIG_PREEMPT
> > doesn't record the number of spinlocks currently taken.
> 
> Ahh right...that makes sense.
> 
> Al also suggested on IRC that we could add a kvfree_atomic if that were
> useful. That might be good for new callers, but we'd probably need a
> patch like this one to suss out which of the existing kvfree callers
> would need to switch to using it.
> 
> I think you're quite right that this is a landmine. That said, this
> seems like something we ought to try to clean up.

I'd rather add a kvfree_fast().  So something like this:

diff --git a/mm/util.c b/mm/util.c
index bab284d69c8c..992f0332dced 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -470,6 +470,28 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
 }
 EXPORT_SYMBOL(kvmalloc_node);
 
+/**
+ * kvfree_fast() - Free memory.
+ * @addr: Pointer to allocated memory.
+ *
+ * kvfree_fast frees memory allocated by any of vmalloc(), kmalloc() or
+ * kvmalloc().  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.  Must not
+ * hold a spinlock as it can sleep.
+ */
+void kvfree_fast(const void *addr)
+{
+	might_sleep();
+
+	if (is_vmalloc_addr(addr))
+		vfree(addr);
+	else
+		kfree(addr);
+}
+EXPORT_SYMBOL(kvfree_fast);
+
 /**
  * kvfree() - Free memory.
  * @addr: Pointer to allocated memory.
@@ -478,12 +500,12 @@ 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.
  */
 void kvfree(const void *addr)
 {
 	if (is_vmalloc_addr(addr))
-		vfree(addr);
+		vfree_atomic(addr);
 	else
 		kfree(addr);
 }