Re: [PATCH] mm/mremap: fix BUILD_BUG_ON() error in get_extent

[Nathan Chancellor <natechancellor@gmail.com>]

On Wed, Dec 30, 2020 at 04:40:40PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> clang cannt evaluate this function argument at compile time
> when the function is not inlined, which leads to a link
> time failure:
> 
> ld.lld: error: undefined symbol: __compiletime_assert_414
> >>> referenced by mremap.c
> >>>               mremap.o:(get_extent) in archive mm/built-in.a
> 
> Mark the function as __always_inline to avoid it.
> 
> Fixes: 9ad9718bfa41 ("mm/mremap: calculate extent in one place")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  mm/mremap.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index c5590afe7165..1cb464a07184 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -336,8 +336,9 @@ enum pgt_entry {
>   * valid. Else returns a smaller extent bounded by the end of the source and
>   * destination pgt_entry.
>   */
> -static unsigned long get_extent(enum pgt_entry entry, unsigned long old_addr,
> -			unsigned long old_end, unsigned long new_addr)
> +static __always_inline unsigned long get_extent(enum pgt_entry entry,
> +			unsigned long old_addr, unsigned long old_end,
> +			unsigned long new_addr)
>  {
>  	unsigned long next, extent, mask, size;
>  
> -- 
> 2.29.2
> 

I am in agreement with Vlastimil, I would rather see the BUILD_BUG()
dropped or converted into BUG() instead of papering over with
__always_inline. For what it's worth, I only see this build failure
with CONFIG_UBSAN_UNSIGNED_OVERFLOW, which you proposed disabling:

https://lore.kernel.org/lkml/20201230154749.746641-1-arnd@kernel.org/

Cheers,
Nathan