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Date:   Fri, 14 Dec 2018 20:23:15 +0100
From:   Gerald Schaefer <gerald.schaefer@de.ibm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Mikhail Zaslonko <zaslonko@linux.ibm.com>,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, mhocko@kernel.org,
        Pavel.Tatashin@microsoft.com, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com
Subject: Re: [PATCH v2 1/1] mm, memory_hotplug: Initialize struct pages for
 the full memory section
In-Reply-To: <bcd0c49c-e417-ef8b-996f-99ecef540d9c@redhat.com>
References: <20181212172712.34019-1-zaslonko@linux.ibm.com>
        <20181212172712.34019-2-zaslonko@linux.ibm.com>
        <476a80cb-5524-16c1-6dd5-da5febbd6139@redhat.com>
        <bcd0c49c-e417-ef8b-996f-99ecef540d9c@redhat.com>
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On Fri, 14 Dec 2018 16:49:14 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 14.12.18 16:22, David Hildenbrand wrote:
> > On 12.12.18 18:27, Mikhail Zaslonko wrote:  
> >> If memory end is not aligned with the sparse memory section boundary, the
> >> mapping of such a section is only partly initialized. This may lead to
> >> VM_BUG_ON due to uninitialized struct page access from
> >> is_mem_section_removable() or test_pages_in_a_zone() function triggered by
> >> memory_hotplug sysfs handlers:
> >>
> >> Here are the the panic examples:
> >>  CONFIG_DEBUG_VM=y
> >>  CONFIG_DEBUG_VM_PGFLAGS=y
> >>
> >>  kernel parameter mem=2050M
> >>  --------------------------
> >>  page:000003d082008000 is uninitialized and poisoned
> >>  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
> >>  Call Trace:
> >>  ([<0000000000385b26>] test_pages_in_a_zone+0xde/0x160)
> >>   [<00000000008f15c4>] show_valid_zones+0x5c/0x190
> >>   [<00000000008cf9c4>] dev_attr_show+0x34/0x70
> >>   [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
> >>   [<00000000003e4194>] seq_read+0x204/0x480
> >>   [<00000000003b53ea>] __vfs_read+0x32/0x178
> >>   [<00000000003b55b2>] vfs_read+0x82/0x138
> >>   [<00000000003b5be2>] ksys_read+0x5a/0xb0
> >>   [<0000000000b86ba0>] system_call+0xdc/0x2d8
> >>  Last Breaking-Event-Address:
> >>   [<0000000000385b26>] test_pages_in_a_zone+0xde/0x160
> >>  Kernel panic - not syncing: Fatal exception: panic_on_oops
> >>
> >>  kernel parameter mem=3075M
> >>  --------------------------
> >>  page:000003d08300c000 is uninitialized and poisoned
> >>  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
> >>  Call Trace:
> >>  ([<000000000038596c>] is_mem_section_removable+0xb4/0x190)
> >>   [<00000000008f12fa>] show_mem_removable+0x9a/0xd8
> >>   [<00000000008cf9c4>] dev_attr_show+0x34/0x70
> >>   [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
> >>   [<00000000003e4194>] seq_read+0x204/0x480
> >>   [<00000000003b53ea>] __vfs_read+0x32/0x178
> >>   [<00000000003b55b2>] vfs_read+0x82/0x138
> >>   [<00000000003b5be2>] ksys_read+0x5a/0xb0
> >>   [<0000000000b86ba0>] system_call+0xdc/0x2d8
> >>  Last Breaking-Event-Address:
> >>   [<000000000038596c>] is_mem_section_removable+0xb4/0x190
> >>  Kernel panic - not syncing: Fatal exception: panic_on_oops
> >>
> >> Fix the problem by initializing the last memory section of each zone
> >> in memmap_init_zone() till the very end, even if it goes beyond the zone
> >> end.
> >>
> >> Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> >> Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
> >> Cc: <stable@vger.kernel.org>
> >> ---
> >>  mm/page_alloc.c | 12 ++++++++++++
> >>  1 file changed, 12 insertions(+)
> >>
> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >> index 2ec9cc407216..e2afdb2dc2c5 100644
> >> --- a/mm/page_alloc.c
> >> +++ b/mm/page_alloc.c
> >> @@ -5542,6 +5542,18 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
> >>  			cond_resched();
> >>  		}
> >>  	}
> >> +#ifdef CONFIG_SPARSEMEM
> >> +	/*
> >> +	 * If the zone does not span the rest of the section then
> >> +	 * we should at least initialize those pages. Otherwise we
> >> +	 * could blow up on a poisoned page in some paths which depend
> >> +	 * on full sections being initialized (e.g. memory hotplug).
> >> +	 */
> >> +	while (end_pfn % PAGES_PER_SECTION) {
> >> +		__init_single_page(pfn_to_page(end_pfn), end_pfn, zone, nid);
> >> +		end_pfn++;  
> > 
> > This page will not be marked as PG_reserved - although it is a physical
> > memory gap. Do we care?
> >   
> 
> Hm, or do we even have any idea what this is (e.g. could it also be
> something not a gap)?

In the "mem=" restriction scenario it would be a gap, and probably fall
into the PG_reserved categorization from your recent patch:
 * - Pages falling into physical memory gaps - not IORESOURCE_SYSRAM. Trying
 *   to read/write these pages might end badly. Don't touch!

Not sure if it could be something else. In theory, if it is possible to have
a scenario where memory zones are not section-aligned, then this
end_pfn % PAGES_PER_SECTION part could be part of another zone. But then it
should not matter if the pages get pre-initialized here, with or w/o
PG_reseved, because they should later be properly initialized in their zone.

So marking them as PG_reserved sounds right, especially in the light of your
current PG_reserved clean-up.

> 
> For physical memory gaps within a section, architectures usually exclude
> that memory from getting passed to e.g. the page allocator by
> memblock_reserve().
> 
> Before handing all free pages to the page allocator, all such reserved
> memblocks will be marked reserved.
> 
> But this here seems to be different. We don't have a previous
> memblock_reserve(), because otherwise these pages would have properly
> been initialized already when marking them reserved.

Not sure how memblock_reserve() and struct page initialization are
related, but at least on s390 there is a memblock_reserve() on the range
in question in setup_arch() -> reserve_memory_end(). However, in this
"mem=" scenario, the range is also removed later with memblock_remove()
in setup_memory_end(), because it is beyond memory_end.