Re: [PATCH v6 27/39] kasan, mm: only define ___GFP_SKIP_KASAN_POISON with HW_TAGS

[Andrey Konovalov]

On Wed, Mar 23, 2022 at 2:02 PM Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> wrote:
>
> On 2022-03-23 12:48:29 [+0100], Vlastimil Babka wrote:
> > > +#ifdef CONFIG_KASAN_HW_TAGS
> > >  #define ___GFP_SKIP_KASAN_POISON   0x1000000u
> > > +#else
> > > +#define ___GFP_SKIP_KASAN_POISON   0
> > > +#endif
> > >  #ifdef CONFIG_LOCKDEP
> > >  #define ___GFP_NOLOCKDEP   0x2000000u
> > >  #else
> > > @@ -251,7 +255,9 @@ struct vm_area_struct;
> > >  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
> > >
> > >  /* Room for N __GFP_FOO bits */
> > > -#define __GFP_BITS_SHIFT (25 + IS_ENABLED(CONFIG_LOCKDEP))
> > > +#define __GFP_BITS_SHIFT (24 +                                     \
> > > +                     IS_ENABLED(CONFIG_KASAN_HW_TAGS) +    \
> > > +                     IS_ENABLED(CONFIG_LOCKDEP))
> >
> > This breaks __GFP_NOLOCKDEP, see:
> > https://lore.kernel.org/all/YjoJ4CzB3yfWSV1F@xxxxxxxxxxxxx/
>
> This could work because ___GFP_NOLOCKDEP is still 0x2000000u. In
>         ("kasan, page_alloc: allow skipping memory init for HW_TAGS")
>         https://lore.kernel.org/all/0d53efeff345de7d708e0baa0d8829167772521e.1643047180.git.andreyknvl@xxxxxxxxxx/
>
> This is replaced with 0x8000000u which breaks lockdep.
>
> Sebastian

Hi Sebastian,

Indeed, sorry for breaking lockdep. Thank you for the report!

I wonder what's the proper fix for this. Perhaps, don't hide KASAN GFP
bits under CONFIG_KASAN_HW_TAGS? And then do:

#define __GFP_BITS_SHIFT (27 + IS_ENABLED(CONFIG_LOCKDEP))

Vlastimil, Andrew do you have any preference?

If my suggestion sounds good, Andrew, could you directly apply the
changes? They are needed for these 3 patches:

kasan, page_alloc: allow skipping memory init for HW_TAGS
kasan, page_alloc: allow skipping unpoisoning for HW_TAGS
kasan, mm: only define ___GFP_SKIP_KASAN_POISON with HW_TAGS

As these depend on each other, I can't send separate patches that can
be folded for all 3.

Thanks!