Re: [PATCH v2 09/16] rcu/tree: Maintain separate array for vmalloc ptrs

Uladzislau Rezki
2020-06-18T18:48:57Z

On Thu, Jun 18, 2020 at 11:37:51AM -0700, Matthew Wilcox wrote:
> On Thu, Jun 18, 2020 at 08:23:33PM +0200, Uladzislau Rezki wrote:
> > > +void vfree_bulk(size_t count, void **addrs)
> > > +{
> > > + unsigned int i;
> > > +
> > > + BUG_ON(in_nmi());
> > > + might_sleep_if(!in_interrupt());
> > > +
> > > + for (i = 0; i < count; i++) {
> > > + void *addr = addrs[i];
> > > + kmemleak_free(addr);
> > > + if (addr)
> > > + __vfree(addr);
> > > + }
> > > +}
> > > +EXPORT_SYMBOL(vfree_bulk);
> > > +
> > >
> > Can we just do addrs[i] all over the loop?
> >
> > Also, we can just call vfree() instead that has all checking we
> > need: NMI, kmemleak, might_sleep.
>
> Of course we _can_. But would we want to? This way, we only do these
> checks once instead of once per pointer, which is rather the point
> of batching.
>
Ahh, right. I briefly looked at it and missed that point. Right you
are we do not want the vfree() here!

>
> I might actually go further and hoist the in_interrupt() check into
> this function ...
>
Why do you need it? Just to inline below code:

<snip>
if (unlikely(in_interrupt()))
__vfree_deferred(addr);
else
__vunmap(addr, 1);
<snip>

and bypass the __vfree() call(that is not marked as inline one)?
I mean to inline above into vfree_bulk().

>
> I suspect the RCU code always runs in_interrupt()
> and so we always call vfree_deferred().
>
No. We release the memory from workqueue context.

--
Vlad Rezki