[PATCH] mm/debug: Use BUILD_BUG_ON_INVALID() for VIRTUAL_BUG_ON()

[PATCH] mm/debug: Use BUILD_BUG_ON_INVALID() for VIRTUAL_BUG_ON()

[Tal Zussman]

This allows the compiler to validate the condition even with
CONFIG_DEBUG_VIRTUAL disabled, and aligns VIRTUAL_BUG_ON() with the
other macros in mmdebug.h.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/mmdebug.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
index a0a3894900ed..012aef40e7a9 100644
--- a/include/linux/mmdebug.h
+++ b/include/linux/mmdebug.h
@@ -129,7 +129,7 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi);
 #ifdef CONFIG_DEBUG_VIRTUAL
 #define VIRTUAL_BUG_ON(cond) BUG_ON(cond)
 #else
-#define VIRTUAL_BUG_ON(cond) do { } while (0)
+#define VIRTUAL_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
 #endif
 
 #ifdef CONFIG_DEBUG_VM_PGFLAGS

---
base-commit: efe99fabeb11b030c89a7dc5a5e7a7558d0dc7ec
change-id: 20250607-virtual_bug_on_invalid-81d24b276109

Best regards,
-- 
Tal Zussman <tz2294@columbia.edu>

Re: [PATCH] mm/debug: Use BUILD_BUG_ON_INVALID() for VIRTUAL_BUG_ON()

[David Hildenbrand]

On 07.06.25 09:09, Tal Zussman wrote:
> This allows the compiler to validate the condition even with
> CONFIG_DEBUG_VIRTUAL disabled, and aligns VIRTUAL_BUG_ON() with the
> other macros in mmdebug.h.
> 

In the light of recent discussions, I think we should get rid of 
VIRTUAL_BUG_ON completely.

There are only a hand full of callers, and I am preety sure for most of 
them VM_WARN_ON is a suitable replacement.

> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> ---
>   include/linux/mmdebug.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
> index a0a3894900ed..012aef40e7a9 100644
> --- a/include/linux/mmdebug.h
> +++ b/include/linux/mmdebug.h
> @@ -129,7 +129,7 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi);
>   #ifdef CONFIG_DEBUG_VIRTUAL
>   #define VIRTUAL_BUG_ON(cond) BUG_ON(cond)
>   #else
> -#define VIRTUAL_BUG_ON(cond) do { } while (0)
> +#define VIRTUAL_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
>   #endif
>   
>   #ifdef CONFIG_DEBUG_VM_PGFLAGS
> 
> ---
> base-commit: efe99fabeb11b030c89a7dc5a5e7a7558d0dc7ec
> change-id: 20250607-virtual_bug_on_invalid-81d24b276109
> 
> Best regards,


-- 
Cheers,

David / dhildenb

Re: [PATCH] mm/debug: Use BUILD_BUG_ON_INVALID() for VIRTUAL_BUG_ON()

[Tal Zussman]

On Sat, Jun 7, 2025 at 3:59 AM David Hildenbrand <david@redhat.com> wrote:
> On 07.06.25 09:09, Tal Zussman wrote:
> > This allows the compiler to validate the condition even with
> > CONFIG_DEBUG_VIRTUAL disabled, and aligns VIRTUAL_BUG_ON() with the
> > other macros in mmdebug.h.
> >
>
> In the light of recent discussions, I think we should get rid of
> VIRTUAL_BUG_ON completely.
>
> There are only a hand full of callers, and I am preety sure for most of
> them VM_WARN_ON is a suitable replacement.

Makes sense. However, all of the callers (except for vmalloc) are already
gated by CONFIG_DEBUG_VIRTUAL, which doesn't depend on CONFIG_DEBUG_VM, so
using VM_WARN_ON_ONCE() would break DEBUG_VIRTUAL on its own.

Perhaps it makes sense to convert the non-vmalloc callers to WARN_ON_ONCE()
instead so DEBUG_VIRTUAL still works. The vmalloc caller would then become

        if (IS_ENABLED(CONFIG_DEBUG_VIRTUAL))
                WARN_ON_ONCE(...);

as opposed to VM_WARN_ON_ONCE(), in order to maintain the existing
DEBUG_VIRTUAL behavior.

Alternatively, DEBUG_VIRTUAL could be folded into DEBUG_VM, but that seems
like a slightly more invasive change...

Thanks,
Tal

Re: [PATCH] mm/debug: Use BUILD_BUG_ON_INVALID() for VIRTUAL_BUG_ON()

[David Hildenbrand]

On 07.06.25 18:21, Tal Zussman wrote:
> On Sat, Jun 7, 2025 at 3:59 AM David Hildenbrand <david@redhat.com> wrote:
>> On 07.06.25 09:09, Tal Zussman wrote:
>>> This allows the compiler to validate the condition even with
>>> CONFIG_DEBUG_VIRTUAL disabled, and aligns VIRTUAL_BUG_ON() with the
>>> other macros in mmdebug.h.
>>>
>>
>> In the light of recent discussions, I think we should get rid of
>> VIRTUAL_BUG_ON completely.
>>
>> There are only a hand full of callers, and I am preety sure for most of
>> them VM_WARN_ON is a suitable replacement.
> 
> Makes sense. However, all of the callers (except for vmalloc) are already
> gated by CONFIG_DEBUG_VIRTUAL, which doesn't depend on CONFIG_DEBUG_VM, so
> using VM_WARN_ON_ONCE() would break DEBUG_VIRTUAL on its own.

It should either be folded or made dependent on DEBUG_VM

> 
> Perhaps it makes sense to convert the non-vmalloc callers to WARN_ON_ONCE()
> instead so DEBUG_VIRTUAL still works.
 > The vmalloc caller would then become>
>          if (IS_ENABLED(CONFIG_DEBUG_VIRTUAL))
>                  WARN_ON_ONCE(...);
> 
> as opposed to VM_WARN_ON_ONCE(), in order to maintain the existing
> DEBUG_VIRTUAL behavior.

 From a quick glimpse, I am not even sure Fedora/RHEL enable it in the 
debug config (and if so, probably it's not enabled by mistake)

I think we can make it depend on DEBUG_VM.

> 
> Alternatively, DEBUG_VIRTUAL could be folded into DEBUG_VM, but that seems
> like a slightly more invasive change...

I'd start with making it depend on DEBUG_VM.

If there is good reason to not fold it, then all VIRTUAL_BUG_ON should 
be renamed to VIRTUAL_WARN_ON and get defined as VM_WARN_ON.

If there is good reason to fold it, then all VIRTUAL_BUG_ON should be 
replaced by VM_WARN_ON.

The only question is what the performance overhead is. I mean, 
performance with DEBUG_VM is already bad (debug kernels), so not sure if 
we really care that much.

-- 
Cheers,

David / dhildenb