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

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Date:   Tue, 23 Jul 2019 14:05:11 -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 10:55 -0700, Matthew Wilcox wrote:
> On Tue, Jul 23, 2019 at 01:52:36PM -0400, Jeff Layton wrote:
> > On Tue, 2019-07-23 at 09:12 -0400, Jeff Layton wrote:
> > > A lot of callers of kvfree only go down the vfree path under very rare
> > > circumstances, and so may never end up hitting the might_sleep_if in it.
> > > Ensure that when kvfree is called, that it is operating in a context
> > > where it is allowed to sleep.
> > > 
> > > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > > Cc: Luis Henriques <lhenriques@...e.com>
> > > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > > ---
> > >  mm/util.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > 
> > FWIW, I started looking at this after Luis sent me some ceph patches
> > that fixed a few of these problems. I have not done extensive testing
> > with this patch, so maybe consider this an RFC for now.
> > 
> > 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.
-- 
Jeff Layton <jlayton@...nel.org>