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Subject: Re: [PATCH] mm/memory-failure: fix crash in split_huge_page_to_list
 from soft_offline_page
To: Matthew Wilcox <willy@infradead.org>
CC: <naoya.horiguchi@nec.com>, <akpm@linux-foundation.org>,
	<linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
References: <20240120065729.3276395-1-linmiaohe@huawei.com>
 <Zax60-Qu14QFuDod@casper.infradead.org>
 <aaff56ad-7bbe-8aa9-cd18-2ac9f4ea23f4@huawei.com>
 <Za59h0ORxsj3t3GW@casper.infradead.org>
From: Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <a9e02fcf-da11-1f0e-1e71-ce88a93d747b@huawei.com>
Date: Tue, 23 Jan 2024 16:06:09 +0800
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On 2024/1/22 22:36, Matthew Wilcox wrote:
> On Mon, Jan 22, 2024 at 08:57:06PM +0800, Miaohe Lin wrote:
>> On 2024/1/21 10:00, Matthew Wilcox wrote:
>>> On Sat, Jan 20, 2024 at 02:57:29PM +0800, Miaohe Lin wrote:
>>>>  {
>>>> -	/* Soft offline could migrate non-LRU movable pages */
>>>> -	if ((flags & MF_SOFT_OFFLINE) && __PageMovable(page))
>>>> +	/*
>>>> +	 * Soft offline could migrate non-LRU movable pages.
>>>> +	 * Note that page->mapping is overloaded with slab->slab_list or slabs
>>>> +	 * fields which might make slab pages appear like non-LRU movable pages.
>>>> +	 * So __PageMovable() has to be done after PageSlab() is checked.
>>>> +	 */
>>>> +	if ((flags & MF_SOFT_OFFLINE) && !PageSlab(page) && __PageMovable(page))
>>>>  		return true;
>>>>  
>>>>  	return PageLRU(page) || is_free_buddy_page(page);
>>>
>>> I think would make more sense as
>>>
>>> +	if (PageSlab(page))
>>> +		return false;
>>
>> Do you mean add PageSlab check above "if ((flags & MF_SOFT_OFFLINE) && __PageMovable(page))" block
>> so we don't need to add more comment?
> 
> Yes, although not just that we don't need to add a comment.
> Fundamentally, if you see PageSlab, you don't need to test anything
> else, you know it's not migratable.

Yes, this really makes sense.

> 
>> I have a concern that __PageMovable() seems unreliable now if we access page from random context.
>> This might introduce some potential problems. For example, offline_pages() might be stumped with
>> such pages without any progress until signal occurs IIUC:
>>
>>   offline_pages
>>     ..
>>     do {
>>       scan_movable_pages
>>         if (__PageMovable(page)) -- It might be slab page here. ret will also be set to 0.
>>           goto found;
>>       do_migrate_range -- Failed to isolate slab page and retry.
>>     } while (!ret) -- retry since ret is 0.
>>
>> There might be many similar scenes, but I haven't taken them more closely. Maybe these are
>> just dumb problems...
> 
> Yep, lots of places are insufficiently careful about testing
> PageMovable.  This will get fixed with memdescs, but we're a fair way
> from having memdescs ...

I believe memdescs will fix all these mess, but we might need to fix them before memdescs becoming ready as a compromise.

Thanks.

> 
> .
>