Re: [PATCH v2] huge_mm.h: disallow is_huge_zero_folio(NULL)

[Max Kellermann]

On Wed, Aug 27, 2025 at 5:21 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
> > But if somebody really passes NULL, the function should not return
> > true - this isn't the huge zero folio after all!  However, if the
> > `huge_zero_folio` hasn't been allocated yet, it's NULL, and
> > is_huge_zero_folio(NULL) just happens to return true, which is a lie.
>
> Hmm seems like this is a bug under a bug. folio_put_refs() shouldn't be
> passed a folio batch of NULL's.

Agree! That was exactly my point - I was hunting down a bug that
sometimes caused folio_put_refs() to crash, but most of the time not
(when no zero huge page was allocated yet). And this randomness is
what I'd like to get rid of.

> Shouldn't we just put the VM_WARN_ON_ONCE() there?

Agree, but that was the 2/2 patch I dropped after David's objection.

> But I really don't think passing NULL to is_huge_zero_folio() is a valid
> enough situation to justify this?
>
> You've encountered a case where a bug caused folio_put_refs() to be called
> with an invalid parameter, then you're arbitrarily changing
> is_huge_zero_folio() so it would deref the folio and splat.

Actually, my v1 patch did not do that. Instead, it checked whether the
huge zero page was already allocated, in order to make
is_huge_zero_folio(NULL) to reliably return false, because NULL is not
the huge zero page. Then David disagreed and asked me to add
VM_WARN_ON_ONCE() instead.

> I really think the VM_WARN_ON_ONCE() should be in folios_put_refs() based
> on what you've said.

You only disagree with David, but not with me. I'm happy with either
way of dealing with this kind of bug/abuse.

> > +#include <linux/mmdebug.h> // for VM_WARN_ON_ONCE()
>
> Please don't do //.

In Linux-main, there are currently 432 comments documenting #include
lines. This is a pretty common coding style.

> 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.

What would you suggest doing instead, to make the VM_WARN_ON_ONCE()
macro available?

> I've had this kind of thing happen several times before.

I know, #includes in Linux are a big mess. A while ago, I tried to
help clean it up, but my effort was rejected by the kernel
maintainers. Which is a pity.