mm/kmemleak: avoid scanning potential huge holes

In the Linux kernel, the following vulnerability has been resolved:

mm/kmemleak: avoid scanning potential huge holes

When using devm_request_free_mem_region() and devm_memremap_pages() to
add ZONE_DEVICE memory, if requested free mem region's end pfn were
huge(e.g., 0x400000000), the node_end_pfn() will be also huge (see
move_pfn_range_to_zone()). Thus it creates a huge hole between
node_start_pfn() and node_end_pfn().

We found on some AMD APUs, amdkfd requested such a free mem region and
created a huge hole. In such a case, following code snippet was just
doing busy test_bit() looping on the huge hole.

for (pfn = start_pfn; pfn < end_pfn; pfn++) {
struct page *page = pfn_to_online_page(pfn);
if (!page)
continue;
...
}

So we got a soft lockup:

watchdog: BUG: soft lockup - CPU#6 stuck for 26s! [bash:1221]
CPU: 6 PID: 1221 Comm: bash Not tainted 5.15.0-custom #1
RIP: 0010:pfn_to_online_page+0x5/0xd0
Call Trace:
? kmemleak_scan+0x16a/0x440
kmemleak_write+0x306/0x3a0
? common_file_perm+0x72/0x170
full_proxy_write+0x5c/0x90
vfs_write+0xb9/0x260
ksys_write+0x67/0xe0
__x64_sys_write+0x1a/0x20
do_syscall_64+0x3b/0xc0
entry_SYSCALL_64_after_hwframe+0x44/0xae

I did some tests with the patch.

(1) amdgpu module unloaded

before the patch:

real 0m0.976s
user 0m0.000s
sys 0m0.968s

after the patch:

real 0m0.981s
user 0m0.000s
sys 0m0.973s

(2) amdgpu module loaded

before the patch:

real 0m35.365s
user 0m0.000s
sys 0m35.354s

after the patch:

real 0m1.049s
user 0m0.000s
sys 0m1.042s