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

Petr Tesařík
2023-05-02T12:50:40Z

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.

> #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).

Petr T