linux-kernel - Re: [PATCH v2 06/39] mm: enumerate all gfp flags

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Message-ID: <20231028192147.2a755c46@meshulam.tesarici.cz>
Date:   Sat, 28 Oct 2023 19:21:47 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Neil Brown <neilb@...e.de>, akpm@...ux-foundation.org,
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Subject: Re: [PATCH v2 06/39] mm: enumerate all gfp flags

On Wed, 25 Oct 2023 08:28:32 -0700
Suren Baghdasaryan <surenb@...gle.com> wrote:

> 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.

You're right. If something does go wrong, it will be easier to fix if
the removal of the unused bit is in a commit of its own.

Petr T