linux-kernel - Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

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Message-ID: <20190801074836.GI11627@dhcp22.suse.cz>
Date:   Thu, 1 Aug 2019 09:48:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Rashmica Gupta <rashmica.g@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        pasha.tatashin@...een.com, Jonathan.Cameron@...wei.com,
        anshuman.khandual@....com, Vlastimil Babka <vbabka@...e.cz>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

On Thu 01-08-19 09:39:57, Michal Hocko wrote:
> On Thu 01-08-19 09:31:09, David Hildenbrand wrote:
> > On 01.08.19 09:26, David Hildenbrand wrote:
> [...]
> > > I am not sure about the implications of having
> > > pfn_valid()/pfn_present()/pfn_online() return true but accessing it
> > > results in crashes. (suspend, kdump, whatever other technology touches
> > > online memory)
> > 
> > (oneidea: we could of course go ahead and mark the pages PG_offline
> > before unmapping the pfn range to work around these issues)
> 
> PG_reserved and an elevated reference count should be enough to drive
> any pfn walker out. Pfn walkers shouldn't touch any page unless they
> know and recognize their type.

Btw. this shouldn't be much different from DEBUG_PAGE_ALLOC in
principle. The memory is valid, but not mapped to the kernel virtual
space. Nobody should be really touching it anyway.
-- 
Michal Hocko
SUSE Labs