Re: [PATCH 09/40] mm: introduce __GFP_NO_OBJ_EXT flag to selectively prevent slabobj_ext creation

Suren Baghdasaryan
2023-05-02T18:33:35Z

On Tue, May 2, 2023 at 5:50 AM Petr Tesařík <petr@tesarici.cz> wrote:
>
> On Mon, 1 May 2023 09:54:19 -0700
> Suren Baghdasaryan <surenb@google.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@google.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