linux-kernel - Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized memmaps in memory

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Message-Id: <20191018190531.975b70fabdce5f7e5d6b27df@linux-foundation.org>
Date:   Fri, 18 Oct 2019 19:05:31 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized
 memmaps in memory_failure()

On Thu, 10 Oct 2019 09:17:42 +0200 David Hildenbrand <david@...hat.com> wrote:

> >> -	pgmap = get_dev_pagemap(pfn, NULL);
> >> -	if (pgmap)
> >> -		return memory_failure_dev_pagemap(pfn, flags, pgmap);
> >> -
> >> -	p = pfn_to_page(pfn);
> > 
> > This change seems to assume that memory_failure_dev_pagemap() is never
> > called for online pages. Is it an intended behavior?
> > Or the concept "online pages" is not applicable to zone device pages?
> 
> Yes, that's the real culprit. ZONE_DEVICE/devmem pages are never online
> (SECTION_IS_ONLINE). The terminology "online" only applies to pages that
> were given to the buddy. And as we support sup-section hotadd for
> devmem, we cannot easily make use of the section flag it. I already
> proposed somewhere to convert SECTION_IS_ONLINE to a subsection bitmap
> and call it something like pfn_active().
> 
> pfn_online() would then be "pfn_active() && zone != ZONE_DEVICE". And we
> could use pfn_active() everywhere to test for initialized memmaps (well,
> besides some special cases like device reserved memory that does not
> span full sub-sections). Until now, nobody volunteered and I have other
> things to do.

Is it worth a code comment or two to make this clearer?