linux-kernel - Re: [PATCH v1 1/2] mm/page_alloc.c: Don't set pages PageReserved() when offlining

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Message-ID: <147fa325-16e3-d2e6-af5c-4cef258c120f@redhat.com>
Date:   Tue, 22 Oct 2019 11:03:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>
Subject: Re: [PATCH v1 1/2] mm/page_alloc.c: Don't set pages PageReserved()
 when offlining

>> - and I remember that PG_reserved on memory holes is relevant to
>> detect MMIO pages. (e.g., looking at KVM code ...)
> 
> I can see kvm_is_reserved_pfn() which checks both pfn_valid and
> PageReserved. How does this help to detect memory holes though?
> Any driver might be setting the page reserved.

See my other mail. This is mostly to not touch MMIO pages and 
ZONE_DEVICE pages ... well and /dev/mem mapped pages.

>   
>>>>> Also is the hole inside a hotplugable memory something we really have to
>>>>> care about. Has anybody actually seen a platform to require that?
>>>>
>>>> That's what I was asking. I can see "support" for this was added basically
>>>> right from the beginning. I'd say we rip that out and cleanup/simplify. I am
>>>> not aware of a platform that requires this. Especially, memory holes on
>>>> DIMMs (detected during boot) seem like an unlikely thing.
>>>
>>> The thing is that the hotplug development shows ad-hoc decisions
>>> throughout the code. It is even worse that it is hard to guess whether
>>> some hludges are a result of a careful design or ad-hoc trial and
>>> failure approach on setups that never were production. Building on top
>>> of that be preserving hacks is not going to improve the situation. So I
>>> am perfectly fine to focus on making the most straightforward setups
>>> work reliably. Even when there is a risk of breaking some odd setups. We
>>> can fix them up later but we would have at least a specific example and
>>> document it.
>>>
>>
>> Alright, I'll prepare a simple patch that rejects offlining memory with
> 
> Is offlining an interesting path? I would expect onlining to be much
> more interesting one.

If you can't offline memory with holes, you can also not online memory 
with holes AFAIKS :)

Bootmem is online, and memory you can hotplug (initially offline) cannot 
have any holes.

-- 

Thanks,

David / dhildenb