arm64: enable THP_SWAP for arm64

arm64: enable THP_SWAP for arm64

THP_SWAP has been proven to improve the swap throughput significantly
on x86_64 according to commit bd4c82c22c367e ("mm, THP, swap: delay
splitting THP after swapped out").
As long as arm64 uses 4K page size, it is quite similar with x86_64
by having 2MB PMD THP. THP_SWAP is architecture-independent, thus,
enabling it on arm64 will benefit arm64 as well.
A corner case is that MTE has an assumption that only base pages
can be swapped. We won't enable THP_SWAP for ARM64 hardware with
MTE support until MTE is reworked to coexist with THP_SWAP.

A micro-benchmark is written to measure thp swapout throughput as
below,

 unsigned long long tv_to_ms(struct timeval tv)
 {
 	return tv.tv_sec * 1000 + tv.tv_usec / 1000;
 }

 main()
 {
 	struct timeval tv_b, tv_e;;
 #define SIZE 400*1024*1024
 	volatile void *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
 				MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
 	if (!p) {
 		perror("fail to get memory");
 		exit(-1);
 	}

 	madvise(p, SIZE, MADV_HUGEPAGE);
 	memset(p, 0x11, SIZE); /* write to get mem */

 	gettimeofday(&tv_b, NULL);
 	madvise(p, SIZE, MADV_PAGEOUT);
 	gettimeofday(&tv_e, NULL);

 	printf("swp out bandwidth: %ld bytes/ms\n",
 			SIZE/(tv_to_ms(tv_e) - tv_to_ms(tv_b)));
 }

Testing is done on rk3568 64bit Quad Core Cortex-A55 platform -
ROCK 3A.
thp swp throughput w/o patch: 2734bytes/ms (mean of 10 tests)
thp swp throughput w/  patch: 3331bytes/ms (mean of 10 tests)

Cc: "Huang, Ying" <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Steven Price <[email protected]>
Cc: Yang Shi <[email protected]>
Reviewed-by: Anshuman Khandual <[email protected]>
Signed-off-by: Barry Song <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>