From: Matthew Wilcox <willy@infradead.org>
To: Jeff Layton <jlayton@kernel.org>
Cc: Alexander Duyck <alexander.duyck@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Luis Henriques <lhenriques@suse.com>,
Christoph Hellwig <hch@lst.de>,
Carlos Maiolino <cmaiolino@redhat.com>
Subject: Re: [PATCH] mm: Make kvfree safe to call
Date: Fri, 26 Jul 2019 17:38:51 -0700 [thread overview]
Message-ID: <20190727003851.GJ30641@bombadil.infradead.org> (raw)
In-Reply-To: <e4b0d323ed0bc159d863945251cf3f4c4064526c.camel@kernel.org>
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@infradead.org> wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@infradead.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@kernel.org>
> > > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> > > Cc: Luis Henriques <lhenriques@suse.com>
> > > Cc: Christoph Hellwig <hch@lst.de>
> > > Cc: Carlos Maiolino <cmaiolino@redhat.com>
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.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 ...
next prev parent reply other threads:[~2019-07-27 0:38 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-26 21:01 [PATCH] mm: Make kvfree safe to call Matthew Wilcox
2019-07-26 21:10 ` Alexander Duyck
2019-07-26 21:25 ` Jeff Layton
2019-07-27 0:38 ` Matthew Wilcox [this message]
2019-07-27 14:54 ` Jeff Layton
2019-07-29 9:28 ` Michal Hocko
2019-07-29 13:42 ` Jason Gunthorpe
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