linux-kernel - Re: [PATCH v2] huge_mm.h: disallow is_huge_zero

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Message-ID: <6e6605f7-0249-4c5d-afac-e85e8536b0b6@lucifer.local>
Date: Wed, 27 Aug 2025 21:44:28 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Max Kellermann <max.kellermann@...os.com>, akpm@...ux-foundation.org,
        ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
        baohua@...nel.org, shikemeng@...weicloud.com, kasong@...cent.com,
        nphamcs@...il.com, bhe@...hat.com, chrisl@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] huge_mm.h: disallow is_huge_zero_folio(NULL)

On Wed, Aug 27, 2025 at 10:26:09PM +0200, David Hildenbrand wrote:
> On 27.08.25 22:01, David Hildenbrand wrote:
> >
> > > This seems not so great.
> > >
> > > I really think the VM_WARN_ON_ONCE() should be in folios_put_refs() based
> > > on what you've said.
> >
> > No.
> >
> > Everything in folio_put_refs() will dereference the folio and properly
> > crash the machine when doing something stupid.
> >
> > This function, however, will silently swallow a "NULL" pointer.
> >
> > >
> > > >
> > > > Signed-off-by: Max Kellermann <max.kellermann@...os.com>
> > > > ---
> > > >    include/linux/huge_mm.h | 3 +++
> > > >    1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > > index 7748489fde1b..bc9ca7798f2e 100644
> > > > --- a/include/linux/huge_mm.h
> > > > +++ b/include/linux/huge_mm.h
> > > > @@ -2,6 +2,7 @@
> > > >    #ifndef _LINUX_HUGE_MM_H
> > > >    #define _LINUX_HUGE_MM_H
> > > >
> > > > +#include <linux/mmdebug.h> // for VM_WARN_ON_ONCE()
> > >
> > > Please don't do //.
> > >
> > > This include is suspect though, huge_mm.h is included from mm.h and thus
> > > this very easily might break some arch that is weird about this stuff,
> > > because a ton of stuff includes mm.h including things that might absolutely
> > > baulk at mmdebug.
> >
> > Jup. Very likely this is not required.
>
> Took another look and including mmdebug.h should likely be fine, because all
> it includes is really
>
> (1) include/linux/bug.h
> (2) include/linux/stringify.h
>
> But originally, I wonder if we even need mmdebug.h or if it is already
> implicitly included.
>
> huge_mm.h includes mm_types.h (where we already do use VM_WARN...).
>
> I assume there we get it implicitly through percpu.h, mmap_lock.h or
> percpu.h
>
> So the include should just get dropped.

Ah nice, let's do this then please! :)

Thanks for checking!

Cheers, Lorenzo