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Date:   Mon, 30 May 2022 08:50:01 +0200
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To:     zhenwei pi <pizhenwei@bytedance.com>, akpm@linux-foundation.org,
        naoya.horiguchi@nec.com, mst@redhat.com
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        jasowang@redhat.com, virtualization@lists.linux-foundation.org,
        pbonzini@redhat.com, peterx@redhat.com, qemu-devel@nongnu.org
References: <20220520070648.1794132-1-pizhenwei@bytedance.com>
 <20220520070648.1794132-3-pizhenwei@bytedance.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH 2/3] mm/memory-failure.c: support reset PTE during
 unpoison
In-Reply-To: <20220520070648.1794132-3-pizhenwei@bytedance.com>
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On 20.05.22 09:06, zhenwei pi wrote:
> Origianlly, unpoison_memory() is only used by hwpoison-inject, and
> unpoisons a page which is poisoned by hwpoison-inject too. The kernel PTE
> entry has no change during software poison/unpoison.
> 
> On a virtualization platform, it's possible to fix hardware corrupted page
> by hypervisor, typically the hypervisor remaps the error HVA(host virtual
> address). So add a new parameter 'const char *reason' to show the reason
> called by.
> 
> Once the corrupted page gets fixed, the guest kernel needs put page to
> buddy. Reuse the page and hit the following issue(Intel Platinum 8260):
>  BUG: unable to handle page fault for address: ffff888061646000
>  #PF: supervisor write access in kernel mode
>  #PF: error_code(0x0002) - not-present page
>  PGD 2c01067 P4D 2c01067 PUD 61aaa063 PMD 10089b063 PTE 800fffff9e9b9062
>  Oops: 0002 [#1] PREEMPT SMP NOPTI
>  CPU: 2 PID: 31106 Comm: stress Kdump: loaded Tainted: G   M       OE     5.18.0-rc6.bm.1-amd64 #6
>  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
>  RIP: 0010:clear_page_erms+0x7/0x10
> 
> The kernel PTE entry of the fixed page is still uncorrected, kernel hits
> page fault during prep_new_page. So add 'bool reset_kpte' to get a change
> to fix the PTE entry if the page is fixed by hypervisor.

Why don't we want to do that for the hwpoison case?

> 
> Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
> ---
>  include/linux/mm.h   |  2 +-
>  mm/hwpoison-inject.c |  2 +-
>  mm/memory-failure.c  | 26 +++++++++++++++++++-------
>  3 files changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 665873c2788c..7ba210e86401 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3191,7 +3191,7 @@ enum mf_flags {
>  extern int memory_failure(unsigned long pfn, int flags);
>  extern void memory_failure_queue(unsigned long pfn, int flags);
>  extern void memory_failure_queue_kick(int cpu);
> -extern int unpoison_memory(unsigned long pfn);
> +extern int unpoison_memory(unsigned long pfn, bool reset_kpte, const char *reason);
>  extern int sysctl_memory_failure_early_kill;
>  extern int sysctl_memory_failure_recovery;
>  extern void shake_page(struct page *p);
> diff --git a/mm/hwpoison-inject.c b/mm/hwpoison-inject.c
> index 5c0cddd81505..0dd17ba98ade 100644
> --- a/mm/hwpoison-inject.c
> +++ b/mm/hwpoison-inject.c
> @@ -57,7 +57,7 @@ static int hwpoison_unpoison(void *data, u64 val)
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
>  
> -	return unpoison_memory(val);
> +	return unpoison_memory(val, false, "hwpoison-inject");

s/hwpoison-inject/hwpoison/

or maybe

s/hwpoison-inject/debugfs/

>  }
>  
>  DEFINE_DEBUGFS_ATTRIBUTE(hwpoison_fops, NULL, hwpoison_inject, "%lli\n");
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 95c218bb0a37..a46de3be1dd7 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -2132,21 +2132,26 @@ core_initcall(memory_failure_init);
>  /**
>   * unpoison_memory - Unpoison a previously poisoned page
>   * @pfn: Page number of the to be unpoisoned page
> + * @reset_kpte: Reset the PTE entry for kmap
> + * @reason: The callers tells why unpoisoning the page
>   *
> - * Software-unpoison a page that has been poisoned by
> - * memory_failure() earlier.
> + * Unpoison a page that has been poisoned by memory_failure() earlier.
>   *
> - * This is only done on the software-level, so it only works
> - * for linux injected failures, not real hardware failures
> + * For linux injected failures, there is no need to reset PTE entry.
> + * It's possible to fix hardware memory failure on a virtualization platform,
> + * once hypervisor fixes the failure, guest needs put page back to buddy and
> + * reset the PTE entry in kernel.

Why can't we do this unconditionally? Just check if the PTE is poisoned,
and if so, reset it.

-- 
Thanks,

David / dhildenb