linux-kernel - Re: [PATCH v6 04/10] mm/memory_hotplug: Don't access uninitialized memmaps in shrink_zone

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Message-ID: <5a4573de-bd8a-6cd3-55d0-86d503a236fd@redhat.com>
Date:   Mon, 14 Oct 2019 11:32:13 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-ia64@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        x86@...nel.org, Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>
Subject: Re: [PATCH v6 04/10] mm/memory_hotplug: Don't access uninitialized
 memmaps in shrink_zone_span()

On 06.10.19 10:56, David Hildenbrand wrote:
> Let's limit shrinking to !ZONE_DEVICE so we can fix the current code. We
> should never try to touch the memmap of offline sections where we could
> have uninitialized memmaps and could trigger BUGs when calling
> page_to_nid() on poisoned pages.
> 
> There is no reliable way to distinguish an uninitialized memmap from an
> initialized memmap that belongs to ZONE_DEVICE, as we don't have
> anything like SECTION_IS_ONLINE we can use similar to
> pfn_to_online_section() for !ZONE_DEVICE memory. E.g.,
> set_zone_contiguous() similarly relies on pfn_to_online_section() and
> will therefore never set a ZONE_DEVICE zone consecutive. Stopping to
> shrink the ZONE_DEVICE therefore results in no observable changes,
> besides /proc/zoneinfo indicating different boundaries - something we
> can totally live with.
> 
> Before commit d0dc12e86b31 ("mm/memory_hotplug: optimize memory
> hotplug"), the memmap was initialized with 0 and the node with the
> right value. So the zone might be wrong but not garbage. After that
> commit, both the zone and the node will be garbage when touching
> uninitialized memmaps.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Fixes: d0dc12e86b31 ("mm/memory_hotplug: optimize memory hotplug")

@Andrew, can you convert that to

Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded 
memory to zones until online") # visible after d0dc12e86b319

and add

Cc: stable@...r.kernel.org # v4.13+


-- 

Thanks,

David / dhildenb