linux-kernel - Re: [PATCH 3/5] mm/migrate.c: fix potential indeterminate pte entry in migrate_vma_insert

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Message-ID: <eca49322-a5d2-d3b0-d2eb-ee7a5db3a942@huawei.com>
Date:   Tue, 23 Mar 2021 19:28:32 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Alistair Popple <apopple@...dia.com>
CC:     David Hildenbrand <david@...hat.com>, <akpm@...ux-foundation.org>,
        <jglisse@...hat.com>, <shy828301@...il.com>, <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] mm/migrate.c: fix potential indeterminate pte entry
 in migrate_vma_insert_page()

On 2021/3/23 19:07, Alistair Popple wrote:
> On Tuesday, 23 March 2021 9:26:43 PM AEDT David Hildenbrand wrote:
>> On 20.03.21 10:36, Miaohe Lin wrote:
>>> If the zone device page does not belong to un-addressable device memory,
>>> the variable entry will be uninitialized and lead to indeterminate pte
>>> entry ultimately. Fix this unexpectant case and warn about it.
>>
>> s/unexpectant/unexpected/
>>
>>>
>>> Fixes: df6ad69838fc ("mm/device-public-memory: device memory cache 
> coherent with CPU")
>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>> ---
>>>   mm/migrate.c | 7 +++++++
>>>   1 file changed, 7 insertions(+)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 20a3bf75270a..271081b014cb 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -2972,6 +2972,13 @@ static void migrate_vma_insert_page(struct 
> migrate_vma *migrate,
>>>   
>>>   			swp_entry = make_device_private_entry(page, vma->vm_flags & 
> VM_WRITE);
>>>   			entry = swp_entry_to_pte(swp_entry);
>>> +		} else {
>>> +			/*
>>> +			 * For now we only support migrating to un-addressable
>>> +			 * device memory.
>>> +			 */
>>> +			WARN_ON(1);
>>> +			goto abort;
>>
>> Fix it by crashing the kernel with panic_on_warn? :)
>>
>> If this case can actual happen, than no WARN_ON() - rather a 
>> pr_warn_once(). If this case cannot happen, why do we even care (it's 
>> not a fix then)?
> 
> There is also already a check for this case in migrate_vma_pages(). The 
> problem is it happens after the call to migrate_vma_insert_page(). I wonder if 
> instead it would be better just to move the existing check to before that 
> call?
> 

Yes, sounds good! Many thanks for your advice! :)

> >
> .
>