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

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Date:   Mon, 21 Oct 2019 17:54:35 +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

On 21.10.19 17:47, Michal Hocko wrote:
> On Mon 21-10-19 17:39:36, David Hildenbrand wrote:
>> On 21.10.19 16:43, Michal Hocko wrote:
> [...]
>>> We still set PageReserved before onlining pages and that one should be
>>> good to go as well (memmap_init_zone).
>>> Thanks!
>>
>> memmap_init_zone() is called when onlining memory. There, set all pages to
>> reserved right now (on context == MEMMAP_HOTPLUG). We clear PG_reserved when
>> onlining a page to the buddy (e.g., generic_online_page). If we would online
>> a memory block with holes, we would want to keep all such pages
>> (!pfn_valid()) set to reserved. Also, there might be other side effects.
> 
> Isn't it sufficient to have those pages in a poisoned state? They are
> not onlined so their state is basically undefined anyway. I do not see
> how PageReserved makes this any better.

It is what people have been using for a long time. Memory hole -> 
PG_reserved. The memmap is valid, but people want to tell "this here is 
crap, don't look at it".

> 
> 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.


-- 

Thanks,

David / dhildenb