linux-kernel - Re: [PATCH] mm/huge_memory: mark huge_zero

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Date: Tue, 14 May 2024 11:05:57 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: Yang Shi <shy828301@...il.com>
CC: <akpm@...ux-foundation.org>, <nao.horiguchi@...il.com>,
	<xuyu@...ux.alibaba.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/huge_memory: mark huge_zero_folio reserved

On 2024/5/13 23:34, Yang Shi wrote:
> On Fri, May 10, 2024 at 9:31 PM Miaohe Lin <linmiaohe@...wei.com> wrote:
>>
>> 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 unexpectly as it appears like a successfully
>> hwpoisoned folio leading to VM_BUG_ON_PAGE(page_ref_count(page) == 0)
>> when releasing huge_zero_folio.
>>
>> Fix this issue by marking huge_zero_folio reserved. So unpoison_memory()
>> will skip this page. This will make it consistent with ZERO_PAGE case too.
> 
> If I read the code correctly, unpoison_memory() should not dec
> refcount for huge zero page by calling put_page_testzero(). The huge
> zero page's real refcount is actually maintained separately by
> huge_zero_refcount. It is different from the regular refount in struct
> folio, see get_huge_zero_page().

Sure. Huge zero folio should be skipped in unpoison_memory(). It's not supported
anyway. I marked huge_zero_folio reserved in order to let unpoison_memory() skip it
by folio_test_reserved(folio) check. But as David points out, the use of PG_reserve
is limited, so I will find another way to fix the issue.
Thanks.
.