Re: [PATCH] mm/huge_memory: mark huge_zero_folio reserved

[Yang Shi <shy828301@gmail.com>]

On Fri, May 10, 2024 at 9:31 PM Miaohe Lin <linmiaohe@huawei.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().

>
> Fixes: 478d134e9506 ("mm/huge_memory: do not overkill when splitting huge_zero_page")
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> Cc: <stable@vger.kernel.org>
> ---
>  mm/huge_memory.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 317de2afd371..d508ff793145 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -212,6 +212,7 @@ static bool get_huge_zero_page(void)
>                 folio_put(zero_folio);
>                 goto retry;
>         }
> +       __folio_set_reserved(zero_folio);
>         WRITE_ONCE(huge_zero_pfn, folio_pfn(zero_folio));
>
>         /* We take additional reference here. It will be put back by shrinker */
> @@ -264,6 +265,7 @@ static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink,
>                 struct folio *zero_folio = xchg(&huge_zero_folio, NULL);
>                 BUG_ON(zero_folio == NULL);
>                 WRITE_ONCE(huge_zero_pfn, ~0UL);
> +               __folio_clear_reserved(zero_folio);
>                 folio_put(zero_folio);
>                 return HPAGE_PMD_NR;
>         }
> --
> 2.33.0
>