Re: [PATCH] mm: move bad zone checking before getting it

[Lucien Wang]

On Thu, Sep 5, 2024 at 6:22 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 05.09.24 11:52, Wang Yibo wrote:
> > When flags from gfp_zone() has an error combination, VM_BUG_ON() should firt know it before use it.
>
> s/firt/first/
>
> Please break long lines. (checkpatch.pl should have warned you)

I will change it.

>
> >
> > Signed-off-by: Wang Yibo <wangyibo@xxxxxxxxxxxxx>
> > ---
> >   include/linux/gfp.h | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index f53f76e0b17e..ca61b2440ab3 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -133,10 +133,11 @@ static inline enum zone_type gfp_zone(gfp_t flags)
> >   {
> >       enum zone_type z;
> >       int bit = (__force int) (flags & GFP_ZONEMASK);
> > +     VM_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
>
> Better use VM_WARN_ON_ONCE() instead while at it.

Although I do not know what results BAD ZONE flags combinations will
cause, I think maybe a BUG is necessary for security ?

>
> >
> >       z = (GFP_ZONE_TABLE >> (bit * GFP_ZONES_SHIFT)) &
> >                                        ((1 << GFP_ZONES_SHIFT) - 1);
> > -     VM_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
> > +
>
> Unrelated whitespace change.

I will change it.

>
> >       return z;
> >   }
> >
>
> But I don't see why we would want this change? It's not like the kernel
> would crash when calculating z.
>
> Or is there some change in behavior I am missing?

I just think flags checking should precede using it logically, when I
review these partial codes. So I submit this patch, there is no other
reason.

>
> --
> Cheers,
>
> David / dhildenb
>