netdev - Re: [PATCH v2 02/24] netmem: Add utility functions

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Date:   Fri, 6 Jan 2023 20:35:08 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     kernel test robot <lkp@...el.com>
Cc:     Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        llvm@...ts.linux.dev, oe-kbuild-all@...ts.linux.dev,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH v2 02/24] netmem: Add utility functions

On Fri, Jan 06, 2023 at 10:24:30AM +0800, kernel test robot wrote:
> >> include/net/page_pool.h:111:21: warning: due to lvalue conversion of the controlling expression, association of type 'const struct netmem' will never be selected because it is qualified [-Wunreachable-code-generic-assoc]
>            return page_to_pfn(netmem_page(nmem));
>                               ^
>    include/net/page_pool.h:100:8: note: expanded from macro 'netmem_page'
>            const struct netmem:    (const struct page *)nmem,              \
>                  ^

OK, figured out what this error means.

#define netmem_page(nmem) (_Generic((*nmem),                            \
        const struct netmem:    (const struct page *)nmem,              \
        struct netmem:          (struct page *)nmem))

Because I defined this with _Generic((*nmem),...) instead of
_Generic((nmem),...) (like page_folio() is defined), clang always
selects the second case and not the const case.  Apparently lvalue
coversions remove the const (backed up by
https://en.cppreference.com/w/c/language/conversion) but I had no idea
that _Generic applied lvalue conversion to the controlling-expression
(it does!  https://en.cppreference.com/w/c/language/generic)

So, yay for clang's extra warning.  I'll fix this up (as below) and send
a v3 next week including the various R-b that I've received.

-#define netmem_page(nmem) (_Generic((*nmem),                           \
-       const struct netmem:    (const struct page *)nmem,              \
-       struct netmem:          (struct page *)nmem))
+#define netmem_page(nmem) (_Generic((nmem),                            \
+       const struct netmem *:  (const struct page *)nmem,              \
+       struct netmem *:        (struct page *)nmem))