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    SubjectRe: [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@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

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