linux-kernel - Re: [PATCH] mm, page_alloc: check pfn is valid before moving to freelist

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Date:   Thu, 14 Apr 2022 16:02:32 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] mm, page_alloc: check pfn is valid before moving to
 freelist

On 13.04.22 22:55, Mike Rapoport wrote:
> On Tue, Apr 12, 2022 at 02:05:51PM -0700, David Rientjes wrote:
>> On Tue, 12 Apr 2022, Andrew Morton wrote:
>>
>>> On Tue, 12 Apr 2022 13:16:23 -0700 Sudarshan Rajagopalan <quic_sudaraja@...cinc.com> wrote:
>>>
>>>> Check if pfn is valid before or not before moving it to freelist.
>>>>
>>>> There are possible scenario where a pageblock can have partial physical
>>>> hole and partial part of System RAM. This happens when base address in RAM
>>>> partition table is not aligned to pageblock size.
>>>>
>>>> ...
>>>>
>>>>
>>>> Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
>>>> Fixes: 4c7b9896621be ("mm: remove pfn_valid_within() and CONFIG_HOLES_IN_ZONE")
>>>
>>> I made that 859a85ddf90e714092dea71a0e54c7b9896621be and added
>>> cc:stable.  I'll await reviewer input before proceeding further.
>>>
>>
>> Acked-by: David Rientjes <rientjes@...gle.com>
>>
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -2521,6 +2521,11 @@ static int move_freepages(struct zone *zone,
>>>>  	int pages_moved = 0;
>>>>  
>>>>  	for (pfn = start_pfn; pfn <= end_pfn;) {
>>>> +		if (!pfn_valid(pfn)) {
>>>
>>> Readers will wonder how we can encounter an invalid pfn here.  A small
>>> comment might help clue them in.
>>>
>>
>> Sudarshan can correct me if I'm wrong, but this has to do with the 
>> pageblock alignment of the caller that assumes all pages in the range has 
>> an underlying struct page that we can access but that fails to hold true 
>> when we have a memory hole.  A comment would definitely help:
> 
> We do have a struct page for every page in a pageblock even if there is a
> hole in the physical memory. If this is not the case, there is more
> fundamental bug that should be fixed.

Also, I dislike handling such a corner case in a way that affects all
other sane cases. move_freepages() is also used for page isolation.

I agree that this should be fixed differently, if possible.

-- 
Thanks,

David / dhildenb