linux-kernel - Re: [PATCH] percpu: improve percpu_alloc

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Date:   Tue, 10 May 2022 07:22:02 +0300
From:   Vasily Averin <vvs@...nvz.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     kernel test robot <lkp@...el.com>, Ingo Molnar <mingo@...hat.com>,
        kbuild-all@...ts.01.org, Shakeel Butt <shakeelb@...gle.com>,
        kernel@...nvz.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>, cgroups@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH] percpu: improve percpu_alloc_percpu event trace

On 5/10/22 00:06, Steven Rostedt wrote:
> I'm curious as to where the gfp_t to unsigned long is happening in the
> macros?

original ___GFP_* flags are usual defines

/* Plain integer GFP bitmasks. Do not use this directly. */
#define ___GFP_DMA              0x01u
#define ___GFP_HIGHMEM          0x02u
#define ___GFP_DMA32            0x04u

... but __GFP_* flags used elsewhere are declared as 'forced to gfp_t'

#define __GFP_DMA       ((__force gfp_t)___GFP_DMA)
#define __GFP_HIGHMEM   ((__force gfp_t)___GFP_HIGHMEM)
#define __GFP_DMA32     ((__force gfp_t)___GFP_DMA32)
...
#define GFP_DMA         __GFP_DMA
#define GFP_DMA32       __GFP_DMA32

... and when  __def_gfpflag_names() traslates them to unsigned long

       {(unsigned long)GFP_DMA,                "GFP_DMA"},             \
       {(unsigned long)__GFP_HIGHMEM,          "__GFP_HIGHMEM"},       \
       {(unsigned long)GFP_DMA32,              "GFP_DMA32"},           \

... it leads to sparse warnings bacuse type gfp_t was declared as 'bitwise'
>From mas sparse

       -Wbitwise
              Warn about unsupported operations or type mismatches with
              restricted integer types.

               Sparse supports an extended attribute,
              __attribute__((bitwise)), which creates a new restricted
              integer type from a base integer type, distinct from the
              base integer type and from any other restricted integer
              type not declared in the same declaration or typedef.

             __bitwise is for *unique types* that cannot be mixed with
              other types, and that you'd never want to just use as a
              random integer (the integer 0 is special, though, and gets
              silently accepted iirc - it's kind of like "NULL" for
              pointers). So "gfp_t" or the "safe endianness" types would
              be __bitwise: you can only operate on them by doing
              specific operations that know about *that* particular
              type.

              Sparse issues these warnings by default.

Thank you,
	Vasily Averin