linux-kernel - Re: [PATCH] mm: Make kvfree safe to call

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Date:   Fri, 26 Jul 2019 17:38:51 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Luis Henriques <lhenriques@...e.com>,
        Christoph Hellwig <hch@....de>,
        Carlos Maiolino <cmaiolino@...hat.com>
Subject: Re: [PATCH] mm: Make kvfree safe to call

On Fri, Jul 26, 2019 at 05:25:03PM -0400, Jeff Layton wrote:
> On Fri, 2019-07-26 at 14:10 -0700, Alexander Duyck wrote:
> > On Fri, Jul 26, 2019 at 2:01 PM Matthew Wilcox <willy@...radead.org> wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> > > 
> > > Since vfree() can sleep, calling kvfree() from contexts where sleeping
> > > is not permitted (eg holding a spinlock) is a bit of a lottery whether
> > > it'll work.  Introduce kvfree_safe() for situations where we know we can
> > > sleep, but make kvfree() safe by default.
> > > 
> > > Reported-by: Jeff Layton <jlayton@...nel.org>
> > > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > > Cc: Luis Henriques <lhenriques@...e.com>
> > > Cc: Christoph Hellwig <hch@....de>
> > > Cc: Carlos Maiolino <cmaiolino@...hat.com>
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > 
> > So you say you are adding kvfree_safe() in the patch description, but
> > it looks like you are introducing kvfree_fast() below. Did something
> > change and the patch description wasn't updated, or is this just the
> > wrong description for this patch?

Oops, bad description.  Thanks, I'll fix it for v2.

> > > +/**
> > > + * 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();
> > > +
> 
>     might_sleep_if(!in_interrupt());
> 
> That's what vfree does anyway, so we might as well exempt the case where
> you are.

True, but if we are in interrupt, then we may as well call kvfree() since
it'll do the same thing, and this way the rules are clearer.

> > > +       if (is_vmalloc_addr(addr))
> > > +               vfree(addr);
> > > +       else
> > > +               kfree(addr);
> > > +}
> > > +EXPORT_SYMBOL(kvfree_fast);
> > > +
> 
> That said -- is this really useful?
> 
> The only way to know that this is safe is to know what sort of
> allocation it is, and in that case you can just call kfree or vfree as
> appropriate.

It's safe if you know you're not holding any spinlocks, for example ...