Re: [PATCH 1/1] mm, memory_hotplug: Initialize struct pages for the full memory section

[Wei Yang]

On Mon, Dec 10, 2018 at 02:07:12PM +0100, 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:
>
> 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
>
>Fix the problem by initializing the last memory section of the highest zone
>in memmap_init_zone() till the very end, even if it goes beyond the zone
>end.
>
>Signed-off-by: Mikhail Zaslonko <zaslonko@xxxxxxxxxxxxx>
>Reviewed-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
>Cc: <stable@xxxxxxxxxxxxxxx>
>---
> mm/page_alloc.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
>diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>index 2ec9cc407216..41ef5508e5f1 100644
>--- a/mm/page_alloc.c
>+++ b/mm/page_alloc.c
>@@ -5542,6 +5542,21 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
> 			cond_resched();
> 		}
> 	}
>+#ifdef CONFIG_SPARSEMEM
>+	/*
>+	 * If there is no zone spanning 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).
>+	 */
>+	if (end_pfn == max_pfn) {
>+		while (end_pfn % PAGES_PER_SECTION) {
>+			__init_single_page(pfn_to_page(end_pfn), end_pfn, zone,
>+					   nid);
>+			end_pfn++;
>+		}
>+	}
>+#endif

If my understanding is correct, end_pfn is not a valid range.

memmap_init_zone() initialize the range [start_pfn, start_pfn + size). I
am afraid this will break the syntax. 

And max_pfn is also not a valid one. For example, on x86,
update_end_of_memory_vars() will update max_pfn, which is calculated by:

    end_pfn = PFN_UP(start + size);

BTW, as you mentioned this apply to hotplug case. And then why this couldn't
happen during boot up? What differ these two cases?

> }
> 
> #ifdef CONFIG_ZONE_DEVICE
>-- 
>2.16.4

-- 
Wei Yang
Help you, Help me