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Date:   Wed, 25 Oct 2023 08:28:32 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Petr Tesařík <petr@...arici.cz>
Cc:     Neil Brown <neilb@...e.de>, akpm@...ux-foundation.org,
        kent.overstreet@...ux.dev, mhocko@...e.com, vbabka@...e.cz,
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        corbet@....net, void@...ifault.com, peterz@...radead.org,
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Subject: Re: [PATCH v2 06/39] mm: enumerate all gfp flags

On Tue, Oct 24, 2023 at 10:47 PM Petr Tesařík <petr@...arici.cz> wrote:
>
> On Tue, 24 Oct 2023 06:46:03 -0700
> Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> > Introduce GFP bits enumeration to let compiler track the number of used
> > bits (which depends on the config options) instead of hardcoding them.
> > That simplifies __GFP_BITS_SHIFT calculation.
> > Suggested-by: Petr Tesařík <petr@...arici.cz>
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > ---
> >  include/linux/gfp_types.h | 90 +++++++++++++++++++++++++++------------
> >  1 file changed, 62 insertions(+), 28 deletions(-)
> >
> > diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
> > index 6583a58670c5..3fbe624763d9 100644
> > --- a/include/linux/gfp_types.h
> > +++ b/include/linux/gfp_types.h
> > @@ -21,44 +21,78 @@ typedef unsigned int __bitwise gfp_t;
> >   * include/trace/events/mmflags.h and tools/perf/builtin-kmem.c
> >   */
> >
> > +enum {
> > +     ___GFP_DMA_BIT,
> > +     ___GFP_HIGHMEM_BIT,
> > +     ___GFP_DMA32_BIT,
> > +     ___GFP_MOVABLE_BIT,
> > +     ___GFP_RECLAIMABLE_BIT,
> > +     ___GFP_HIGH_BIT,
> > +     ___GFP_IO_BIT,
> > +     ___GFP_FS_BIT,
> > +     ___GFP_ZERO_BIT,
> > +     ___GFP_UNUSED_BIT,      /* 0x200u unused */
> > +     ___GFP_DIRECT_RECLAIM_BIT,
> > +     ___GFP_KSWAPD_RECLAIM_BIT,
> > +     ___GFP_WRITE_BIT,
> > +     ___GFP_NOWARN_BIT,
> > +     ___GFP_RETRY_MAYFAIL_BIT,
> > +     ___GFP_NOFAIL_BIT,
> > +     ___GFP_NORETRY_BIT,
> > +     ___GFP_MEMALLOC_BIT,
> > +     ___GFP_COMP_BIT,
> > +     ___GFP_NOMEMALLOC_BIT,
> > +     ___GFP_HARDWALL_BIT,
> > +     ___GFP_THISNODE_BIT,
> > +     ___GFP_ACCOUNT_BIT,
> > +     ___GFP_ZEROTAGS_BIT,
> > +#ifdef CONFIG_KASAN_HW_TAGS
> > +     ___GFP_SKIP_ZERO_BIT,
> > +     ___GFP_SKIP_KASAN_BIT,
> > +#endif
> > +#ifdef CONFIG_LOCKDEP
> > +     ___GFP_NOLOCKDEP_BIT,
> > +#endif
> > +     ___GFP_LAST_BIT
> > +};
> > +
> >  /* Plain integer GFP bitmasks. Do not use this directly. */
> > -#define ___GFP_DMA           0x01u
> > -#define ___GFP_HIGHMEM               0x02u
> > -#define ___GFP_DMA32         0x04u
> > -#define ___GFP_MOVABLE               0x08u
> > -#define ___GFP_RECLAIMABLE   0x10u
> > -#define ___GFP_HIGH          0x20u
> > -#define ___GFP_IO            0x40u
> > -#define ___GFP_FS            0x80u
> > -#define ___GFP_ZERO          0x100u
> > +#define ___GFP_DMA           BIT(___GFP_DMA_BIT)
> > +#define ___GFP_HIGHMEM               BIT(___GFP_HIGHMEM_BIT)
> > +#define ___GFP_DMA32         BIT(___GFP_DMA32_BIT)
> > +#define ___GFP_MOVABLE               BIT(___GFP_MOVABLE_BIT)
> > +#define ___GFP_RECLAIMABLE   BIT(___GFP_RECLAIMABLE_BIT)
> > +#define ___GFP_HIGH          BIT(___GFP_HIGH_BIT)
> > +#define ___GFP_IO            BIT(___GFP_IO_BIT)
> > +#define ___GFP_FS            BIT(___GFP_FS_BIT)
> > +#define ___GFP_ZERO          BIT(___GFP_ZERO_BIT)
> >  /* 0x200u unused */
>
> This comment can be also removed here, because it is already stated
> above with the definition of ___GFP_UNUSED_BIT.

Ack.

>
> Then again, I think that the GFP bits have never been compacted after
> Neil Brown removed __GFP_ATOMIC with commit 2973d8229b78 simply because
> that would mean changing definitions of all subsequent GFP flags. FWIW
> I am not aware of any code that would depend on the numeric value of
> ___GFP_* macros, so this patch seems like a good opportunity to change
> the numbering and get rid of this unused 0x200u altogether.
>
> @Neil: I have added you to the conversation in case you want to correct
> my understanding of the unused bit.

Hmm. I would prefer to do that in a separate patch even though it
would be a one-line change. Seems safer to me in case something goes
wrong and we have to bisect and revert it. If that sounds ok I'll post
that in the next version.

>
> Other than that LGTM.

Thanks for the review!
Suren.

>
> Petr T
>
> > -#define ___GFP_DIRECT_RECLAIM        0x400u
> > -#define ___GFP_KSWAPD_RECLAIM        0x800u
> > -#define ___GFP_WRITE         0x1000u
> > -#define ___GFP_NOWARN                0x2000u
> > -#define ___GFP_RETRY_MAYFAIL 0x4000u
> > -#define ___GFP_NOFAIL                0x8000u
> > -#define ___GFP_NORETRY               0x10000u
> > -#define ___GFP_MEMALLOC              0x20000u
> > -#define ___GFP_COMP          0x40000u
> > -#define ___GFP_NOMEMALLOC    0x80000u
> > -#define ___GFP_HARDWALL              0x100000u
> > -#define ___GFP_THISNODE              0x200000u
> > -#define ___GFP_ACCOUNT               0x400000u
> > -#define ___GFP_ZEROTAGS              0x800000u
> > +#define ___GFP_DIRECT_RECLAIM        BIT(___GFP_DIRECT_RECLAIM_BIT)
> > +#define ___GFP_KSWAPD_RECLAIM        BIT(___GFP_KSWAPD_RECLAIM_BIT)
> > +#define ___GFP_WRITE         BIT(___GFP_WRITE_BIT)
> > +#define ___GFP_NOWARN                BIT(___GFP_NOWARN_BIT)
> > +#define ___GFP_RETRY_MAYFAIL BIT(___GFP_RETRY_MAYFAIL_BIT)
> > +#define ___GFP_NOFAIL                BIT(___GFP_NOFAIL_BIT)
> > +#define ___GFP_NORETRY               BIT(___GFP_NORETRY_BIT)
> > +#define ___GFP_MEMALLOC              BIT(___GFP_MEMALLOC_BIT)
> > +#define ___GFP_COMP          BIT(___GFP_COMP_BIT)
> > +#define ___GFP_NOMEMALLOC    BIT(___GFP_NOMEMALLOC_BIT)
> > +#define ___GFP_HARDWALL              BIT(___GFP_HARDWALL_BIT)
> > +#define ___GFP_THISNODE              BIT(___GFP_THISNODE_BIT)
> > +#define ___GFP_ACCOUNT               BIT(___GFP_ACCOUNT_BIT)
> > +#define ___GFP_ZEROTAGS              BIT(___GFP_ZEROTAGS_BIT)
> >  #ifdef CONFIG_KASAN_HW_TAGS
> > -#define ___GFP_SKIP_ZERO     0x1000000u
> > -#define ___GFP_SKIP_KASAN    0x2000000u
> > +#define ___GFP_SKIP_ZERO     BIT(___GFP_SKIP_ZERO_BIT)
> > +#define ___GFP_SKIP_KASAN    BIT(___GFP_SKIP_KASAN_BIT)
> >  #else
> >  #define ___GFP_SKIP_ZERO     0
> >  #define ___GFP_SKIP_KASAN    0
> >  #endif
> >  #ifdef CONFIG_LOCKDEP
> > -#define ___GFP_NOLOCKDEP     0x4000000u
> > +#define ___GFP_NOLOCKDEP     BIT(___GFP_NOLOCKDEP_BIT)
> >  #else
> >  #define ___GFP_NOLOCKDEP     0
> >  #endif
> > -/* If the above are modified, __GFP_BITS_SHIFT may need updating */
> >
> >  /*
> >   * Physical address zone modifiers (see linux/mmzone.h - low four bits)
> > @@ -249,7 +283,7 @@ 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 ___GFP_LAST_BIT
> >  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
> >
> >  /**
>
>

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