linux-kernel - Re: [PATCH 09/40] mm: introduce __GFP_NO_OBJ_EXT flag to selectively prevent slabobj

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Date:   Tue, 2 May 2023 11:33:15 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Petr Tesařík <petr@...arici.cz>
Cc:     akpm@...ux-foundation.org, kent.overstreet@...ux.dev,
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Subject: Re: [PATCH 09/40] mm: introduce __GFP_NO_OBJ_EXT flag to selectively
 prevent slabobj_ext creation

On Tue, May 2, 2023 at 5:50 AM Petr Tesařík <petr@...arici.cz> wrote:
>
> On Mon,  1 May 2023 09:54:19 -0700
> Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> > Introduce __GFP_NO_OBJ_EXT flag in order to prevent recursive allocations
> > when allocating slabobj_ext on a slab.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > ---
> >  include/linux/gfp_types.h | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
> > index 6583a58670c5..aab1959130f9 100644
> > --- a/include/linux/gfp_types.h
> > +++ b/include/linux/gfp_types.h
> > @@ -53,8 +53,13 @@ typedef unsigned int __bitwise gfp_t;
> >  #define ___GFP_SKIP_ZERO     0
> >  #define ___GFP_SKIP_KASAN    0
> >  #endif
> > +#ifdef CONFIG_SLAB_OBJ_EXT
> > +#define ___GFP_NO_OBJ_EXT       0x4000000u
> > +#else
> > +#define ___GFP_NO_OBJ_EXT       0
> > +#endif
> >  #ifdef CONFIG_LOCKDEP
> > -#define ___GFP_NOLOCKDEP     0x4000000u
> > +#define ___GFP_NOLOCKDEP     0x8000000u
>
> So now we have two flags that depend on config options, but the first
> one is always allocated in fact. I wonder if you could use an enum to
> let the compiler allocate bits. Something similar to what Muchun Song
> did with section flags.
>
> See commit ed7802dd48f7a507213cbb95bb4c6f1fe134eb5d for reference.

Thanks for the reference. I'll take a closer look and will try to clean it up.

>
> >  #else
> >  #define ___GFP_NOLOCKDEP     0
> >  #endif
> > @@ -99,12 +104,15 @@ typedef unsigned int __bitwise gfp_t;
> >   * node with no fallbacks or placement policy enforcements.
> >   *
> >   * %__GFP_ACCOUNT causes the allocation to be accounted to kmemcg.
> > + *
> > + * %__GFP_NO_OBJ_EXT causes slab allocation to have no object
> > extension. */
> >  #define __GFP_RECLAIMABLE ((__force gfp_t)___GFP_RECLAIMABLE)
> >  #define __GFP_WRITE  ((__force gfp_t)___GFP_WRITE)
> >  #define __GFP_HARDWALL   ((__force gfp_t)___GFP_HARDWALL)
> >  #define __GFP_THISNODE       ((__force gfp_t)___GFP_THISNODE)
> >  #define __GFP_ACCOUNT        ((__force gfp_t)___GFP_ACCOUNT)
> > +#define __GFP_NO_OBJ_EXT   ((__force gfp_t)___GFP_NO_OBJ_EXT)
> >
> >  /**
> >   * DOC: Watermark modifiers
> > @@ -249,7 +257,7 @@ typedef unsigned int __bitwise gfp_t;
> >  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
> >
> >  /* Room for N __GFP_FOO bits */
> > -#define __GFP_BITS_SHIFT (26 + IS_ENABLED(CONFIG_LOCKDEP))
> > +#define __GFP_BITS_SHIFT (27 + IS_ENABLED(CONFIG_LOCKDEP))
>
> If the above suggestion is implemented, this could be changed to
> something like __GFP_LAST_BIT (the enum's last identifier).

Ack.

Thanks for reviewing!
Suren.

>
> Petr T