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

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Date:   Thu, 18 Jun 2020 20:48:50 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH v2 09/16] rcu/tree: Maintain separate array for vmalloc
 ptrs

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