linux-kernel - Re: [PATCH] mm,swapops: Update check in is_pfn_swap

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Date: Wed, 10 Apr 2024 16:16:10 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: Oscar Salvador <osalvador@...e.de>, Peter Xu <peterx@...hat.com>
CC: Andrew Morton <akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, David Hildenbrand <david@...hat.com>,
	<stable@...r.kernel.org>, Tony Luck <tony.luck@...el.com>, Naoya Horiguchi
	<naoya.horiguchi@....com>
Subject: Re: [PATCH] mm,swapops: Update check in is_pfn_swap_entry for
 hwpoison entries

On 2024/4/8 4:31, Oscar Salvador wrote:
>> Totally unexpected, as this commit even removed hwpoison_entry_to_pfn().
>> Obviously even until now I assumed hwpoison is accounted as pfn swap entry
>> but it's just missing..
>>
>> Since this commit didn't really change is_pfn_swap_entry() itself, I was
>> thinking maybe an older fix tag would apply, but then I noticed the old
>> code indeed should work well even if hwpoison entry is missing.  For
>> example, it's a grey area on whether a hwpoisoned page should be accounted
>> in smaps.  So I think the Fixes tag is correct, and thanks for fixing this.
>>
>> Reviewed-by: Peter Xu <peterx@...hat.com>
> 
> Thanks Peter

Thanks both.

> 
>> Fedora stopped having DEBUG_VM for some time, but not sure about when it's
>> still in the 6.1 trees.  It looks like cc stable is still reasonable from
>> that regard.
> 
> Good to know, thanks for the info.
> 
>> A side note is that when I'm looking at this, I went back and see why in
>> some cases we need the pfn maintained for the poisoned, then I saw the only
>> user is check_hwpoisoned_entry() who wants to do fast kills in some
>> contexts and that includes a double check on the pfns in a poisoned entry.
>> Then afaict this path is just too rarely used and buggy.
> 
> Yes, unfortunately memory-failure code does not get exercised that much,
> and so there might be subtly bugs lurking in there for quite some time.

There're many memory-failure testcases but some code paths still didn't get
exercised. That's a pity. :(

> 
>> A few things we may need fixing, maybe someone in the loop would have time
>> to have a look:
>>
>> - check_hwpoisoned_entry()
>>   - pte_none check is missing
>>   - all the rest swap types are missing (e.g., we want to kill the proc too
>>     if the page is during migration)

Firstly, I thought rest swap types just won't exist in this code path. But after second thought,
it seems it's possible. For example, when page is being isolated for migration, memory_failure
will fails to isolate it. And the second MCE event will goes to kill_accessing_process() and
see a migrate swap entry.

>> - check_hwpoisoned_pmd_entry()
>>   - need similar care like above (pmd_none is covered not others)
> 
> I will have a look and see what needs fixing, thanks for bringing it up.

Thanks for your time.
.

> 
>