-Wsizeof-array-div in mm/hugetlb.c

[Nathan Chancellor <natechancellor@gmail.com>]

Hi all,

Clang recently added a new diagnostic in r371605, -Wsizeof-array-div,
that tries to warn when sizeof(X) / sizeof(Y) does not compute the
number of elements in an array X (i.e., sizeof(Y) is wrong). See that
commit for more details:

https://github.com/llvm/llvm-project/commit/3240ad4ced0d3223149b72a4fc2a4d9b67589427

There is a warning in mm/hugetlb.c in hugetlb_fault_mutex_hash:

mm/hugetlb.c:4055:40: warning: expression does not compute the number of
elements in this array; element type is 'unsigned long', not 'u32' (aka
'unsigned int') [-Wsizeof-array-div]
        hash = jhash2((u32 *)&key, sizeof(key)/sizeof(u32), 0);
                                          ~~~ ^
mm/hugetlb.c:4049:16: note: array 'key' declared here
        unsigned long key[2];
                      ^
1 warning generated.

Should this warning be silenced? What is the reasoning behind having key
be an array of unsigned longs but representing it as an array of u32s?
Would it be better to avoid the cast and have it just be an array of
u32s directly? I am not familiar with this code so I may be naive for
asking such questions but we'd like to get these warnings cleaned up so
that this warning can be useful down the road.

Cheers,
Nathan