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

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Message-ID: <d7cd46333eb1a29fb7e0e078dc4fef7646fe2a8c.camel@kernel.org>
Date:   Tue, 23 Jul 2019 14:19:03 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
        lhenriques@...e.com, cmaiolino@...hat.com,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] mm: check for sleepable context in kvfree

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:
> > > > HCH points out that xfs uses kvfree as a generic "free this no matter
> > > > what it is" sort of wrapper and expects the callers to work out whether
> > > > they might be freeing a vmalloc'ed address. If that sort of usage turns
> > > > out to be prevalent, then we may need another approach to clean this up.
> > > 
> > > 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.
-- 
Jeff Layton <jlayton@...nel.org>