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

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Date:   Tue, 22 Oct 2019 10:20:53 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
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 Mon 21-10-19 17:54:35, David Hildenbrand wrote:
> 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".

The page is poisoned, right? If yes then setting the reserved bit
doesn't make any sense.

> > 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.
-- 
Michal Hocko
SUSE Labs