mm/huge_memory: don't unpoison huge_zero_folio

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

mm/huge_memory: don't unpoison huge_zero_folio

When I did memory failure tests recently, below panic occurs:

kernel BUG at include/linux/mm.h:1135!

invalid opcode: 0000 [#1] PREEMPT SMP NOPTI

CPU: 9 PID: 137 Comm: kswapd1 Not tainted 6.9.0-rc4-00491-gd5ce28f156fe-dirty #14

RIP: 0010:shrink_huge_zero_page_scan+0x168/0x1a0

RSP: 0018:ffff9933c6c57bd0 EFLAGS: 00000246

RAX: 000000000000003e RBX: 0000000000000000 RCX: ffff88f61fc5c9c8

RDX: 0000000000000000 RSI: 0000000000000027 RDI: ffff88f61fc5c9c0

RBP: ffffcd7c446b0000 R08: ffffffff9a9405f0 R09: 0000000000005492

R10: 00000000000030ea R11: ffffffff9a9405f0 R12: 0000000000000000

R13: 0000000000000000 R14: 0000000000000000 R15: ffff88e703c4ac00

FS: 0000000000000000(0000) GS:ffff88f61fc40000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 000055f4da6e9878 CR3: 0000000c71048000 CR4: 00000000000006f0

Call Trace:

<TASK>

do_shrink_slab+0x14f/0x6a0

shrink_slab+0xca/0x8c0

shrink_node+0x2d0/0x7d0

balance_pgdat+0x33a/0x720

kswapd+0x1f3/0x410

kthread+0xd5/0x100

ret_from_fork+0x2f/0x50

ret_from_fork_asm+0x1a/0x30

</TASK>

Modules linked in: mce_inject hwpoison_inject

---[ end trace 0000000000000000 ]---

RIP: 0010:shrink_huge_zero_page_scan+0x168/0x1a0

RSP: 0018:ffff9933c6c57bd0 EFLAGS: 00000246

RAX: 000000000000003e RBX: 0000000000000000 RCX: ffff88f61fc5c9c8

RDX: 0000000000000000 RSI: 0000000000000027 RDI: ffff88f61fc5c9c0

RBP: ffffcd7c446b0000 R08: ffffffff9a9405f0 R09: 0000000000005492

R10: 00000000000030ea R11: ffffffff9a9405f0 R12: 0000000000000000

R13: 0000000000000000 R14: 0000000000000000 R15: ffff88e703c4ac00

FS: 0000000000000000(0000) GS:ffff88f61fc40000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 000055f4da6e9878 CR3: 0000000c71048000 CR4: 00000000000006f0

The root cause is that HWPoison flag will be set for huge_zero_folio

without increasing the folio refcnt. But then unpoison_memory() will

decrease the folio refcnt unexpectedly as it appears like a successfully

hwpoisoned folio leading to VM_BUG_ON_PAGE(page_ref_count(page) == 0) when

releasing huge_zero_folio.

Skip unpoisoning huge_zero_folio in unpoison_memory() to fix this issue.

We're not prepared to unpoison huge_zero_folio yet.