Without this patch, the MAP_SYNC test case will cause a print_bad_pte
warning on arm64 as follows:

[   25.542693] BUG: Bad page map in process mapdax333 pte:2e8000448800f53 pmd:41ff5f003
[   25.546360] page:ffff7e0010220000 refcount:1 mapcount:-1 mapping:ffff8003e29c7440 index:0x0
[   25.550281] ext4_dax_aops
[   25.550282] name:"__aaabbbcccddd__"
[   25.551553] flags: 0x3ffff0000001002(referenced|reserved)
[   25.555802] raw: 03ffff0000001002 ffff8003dfffa908 0000000000000000 ffff8003e29c7440
[   25.559446] raw: 0000000000000000 0000000000000000 00000001fffffffe 0000000000000000
[   25.563075] page dumped because: bad pte
[   25.564938] addr:0000ffffbe05b000 vm_flags:208000fb anon_vma:0000000000000000 mapping:ffff8003e29c7440 index:0
[   25.574272] file:__aaabbbcccddd__ fault:ext4_dax_fault mmmmap:ext4_file_mmap readpage:0x0
[   25.578799] CPU: 1 PID: 1180 Comm: mapdax333 Not tainted 5.2.0+ #21
[   25.581702] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
[   25.585624] Call trace:
[   25.587008]  dump_backtrace+0x0/0x178
[   25.588799]  show_stack+0x24/0x30
[   25.590328]  dump_stack+0xa8/0xcc
[   25.591901]  print_bad_pte+0x18c/0x218
[   25.593628]  unmap_page_range+0x778/0xc00
[   25.595506]  unmap_single_vma+0x94/0xe8
[   25.597304]  unmap_vmas+0x90/0x108
[   25.598901]  unmap_region+0xc0/0x128
[   25.600566]  __do_munmap+0x284/0x3f0
[   25.602245]  __vm_munmap+0x78/0xe0
[   25.603820]  __arm64_sys_munmap+0x34/0x48
[   25.605709]  el0_svc_common.constprop.0+0x78/0x168
[   25.607956]  el0_svc_handler+0x34/0x90
[   25.609698]  el0_svc+0x8/0xc
[...]

The root cause is in _vm_normal_page, without the PTE_SPECIAL bit,
the return value will be incorrectly set to pfn_to_page(pfn) instead
of NULL. Besides, this patch also rewrite the pmd_mkdevmap to avoid
setting PTE_SPECIAL for pmd

The MAP_SYNC test case is as follows(Provided by Yibo Cai)
$#include <stdio.h>
$#include <string.h>
$#include <unistd.h>
$#include <sys/file.h>
$#include <sys/mman.h>

$#ifndef MAP_SYNC
$#define MAP_SYNC 0x80000
$#endif

/* mount -o dax /dev/pmem0 /mnt */
$#define F "/mnt/__aaabbbcccddd__"

int main(void)
{
    int fd;
    char buf[4096];
    void *addr;

    if ((fd = open(F, O_CREAT|O_TRUNC|O_RDWR, 0644)) < 0) {
        perror("open1");
        return 1;
    }

    if (write(fd, buf, 4096) != 4096) {
        perror("lseek");
        return 1;
    }

    addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_SYNC, fd, 0);
    if (addr == MAP_FAILED) {
        perror("mmap");
        printf("did you mount with '-o dax'?\n");
        return 1;
    }

    memset(addr, 0x55, 4096);

    if (munmap(addr, 4096) == -1) {
        perror("munmap");
        return 1;
    }

    close(fd);

    return 0;
}

Fixes: 73b20c84 ("arm64: mm: implement pte_devmap support")
Reported-by: Yibo Cai <Yibo.Cai@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Robin Murphy <Robin.Murphy@arm.com>
Signed-off-by: Jia He <justin.he@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>