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

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Message-ID: <488547af-3fa0-69bc-1c05-f4a077ab67d1@huawei.com>
Date:   Tue, 23 Mar 2021 19:24:18 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     David Hildenbrand <david@...hat.com>, <akpm@...ux-foundation.org>
CC:     <jglisse@...hat.com>, <shy828301@...il.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 3/5] mm/migrate.c: fix potential indeterminate pte entry
 in migrate_vma_insert_page()

Hi:
On 2021/3/23 18:26, 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? :)
> 

Sorry, my bad. :(

> 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)?

Yep, this case can actual happen. Many thanks for providing alternative pr_warn_once().

> 
>