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Date:   Tue, 2 May 2023 14:50:14 +0200
From:   Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To:     Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH 09/40] mm: introduce __GFP_NO_OBJ_EXT flag to
 selectively prevent slabobj_ext creation
Message-ID: <20230502145014.24b28e64@meshulam.tesarici.cz>
In-Reply-To: <20230501165450.15352-10-surenb@google.com>
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        <20230501165450.15352-10-surenb@google.com>
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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