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Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.
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From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <b02e08bb-b551-e239-6f30-1db4587850c8@linux.intel.com>
Date: Thu, 18 Jan 2018 06:51:37 -0800
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, torvalds@linux-foundation.org, kirill.shutemov@linux.intel.com, akpm@linux-foundation.org, hannes@cmpxchg.org, iamjoonsoo.kim@lge.com, mgorman@techsingularity.net, tony.luck@intel.com, vbabka@suse.cz, mhocko@kernel.org, aarcange@redhat.com, hillf.zj@alibaba-inc.com, hughd@google.com, oleg@redhat.com, peterz@infradead.org, riel@redhat.com, srikar@linux.vnet.ibm.com, vdavydov.dev@gmail.com, mingo@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org

On 01/18/2018 06:45 AM, Kirill A. Shutemov wrote:
> On Thu, Jan 18, 2018 at 06:38:10AM -0800, Dave Hansen wrote:
>> On 01/18/2018 05:12 AM, Kirill A. Shutemov wrote:
>>> -		if (pte_page(*pvmw->pte) - pvmw->page >=
>>> -				hpage_nr_pages(pvmw->page)) {
>> Is ->pte guaranteed to map a page which is within the same section as
>> pvmw->page?  Otherwise, with sparsemem (non-vmemmap), the pointer
>> arithmetic won't work.
> No, it's not guaranteed. It can be arbitrary page.
> 
> The arithmetic won't work because they are different "memory objects"?

No, because sections' mem_map[]s can be allocated non-contiguously.
Section 1's might be a lower virtual address than Section 0's.

They're allocated not unlike this:

	mem_section[0]->section_mem_map = kmalloc(SECTION_SIZE);
	mem_section[1]->section_mem_map = kmalloc(SECTION_SIZE);
	...

The first pfn in section 1 and the last pfn in section 0 are adjacent
PFNs, but their 'struct page' might have virtual addresses that are TB
apart.

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