Re: [RFC 0/3] Regressions due to 7b79d10a2d64 ("mm: convert kmalloc_section_memmap() to populate_section_memmap()") and Kasan initialization on

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 03/10/2017 03:58 AM, Dan Williams wrote:
> On Fri, Mar 3, 2017 at 8:08 AM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>> On 02/25/2017 10:03 PM, Dan Williams wrote:
>>> [ adding kasan folks ]
>>>
>>> On Wed, Feb 15, 2017 at 12:58 PM, Nicolai Stange <nicstange@gmail.com> wrote:
>>>> Hi Dan,
>>>>
>>>> your recent commit 7b79d10a2d64 ("mm: convert kmalloc_section_memmap() to
>>>> populate_section_memmap()") seems to cause some issues with respect to
>>>> Kasan initialization on x86.
>>>>
>>>> This is because Kasan's initialization (ab)uses the arch provided
>>>> vmemmap_populate().
>>>>
>>>> The first one is a boot failure, see [1/3]. The commit before the
>>>> aforementioned one works fine.
>>>>
>>>> The second one, i.e. [2/3], is something that hit my eye while browsing
>>>> the source and I verified that this is indeed an issue by printk'ing and
>>>> dumping the page tables.
>>>>
>>>> The third one are excessive warnings from vmemmap_verify() due to Kasan's
>>>> NUMA_NO_NODE page populations.
>>>>
>>>>
>>>> I'll be travelling the next two days and certainly not be able to respond
>>>> or polish these patches any further. Furthermore, the next merge window is
>>>> close. So please, take these three patches as bug reports only, meant to
>>>> illustrate the issues. Feel free to use, change and adopt them however
>>>> you deemed best.
>>>>
>>>> That being said,
>>>> - [2/3] will break arm64 due to the current lack of a pmd_large().
>>>> - Maybe it's easier and better to restore former behaviour by letting
>>>>   Kasan's shadow initialization on x86 use vmemmap_populate_hugepages()
>>>>   directly rather than vmemmap_populate(). This would require x86_64
>>>>   implying X86_FEATURE_PSE though. I'm not sure whether this holds,
>>>>   in particular not since the vmemmap_populate() from
>>>>   arch/x86/mm/init_64.c checks for it.
>>>
>>> I think your intuition is correct here, and yes, it is a safe
>>> assumption that x86_64 implies X86_FEATURE_PSE. The following patch
>>> works for me. If there's no objections I'll roll it into the series
>>> and resubmit the sub-section hotplug support after testing on top of
>>> 4.11-rc1.
>>>
>>
>> Perhaps it would be better to get rid of vmemmap in kasan code at all
>> and have a separate function that populates kasan shadow.
>> kasan is abusing API designed for something else. We already had bugs on arm64 (see 2776e0e8ef683)
>> because of that and now this one on x86_64.
>> I can cook patches and send them on the next week.
>>
> 
> Any concerns with proceeding with the conversion to explicit
> vmemmap_populate_hugepages() calls in the meantime? That allows me to
> unblock the sub-section hotplug patches and kasan can move away from
> vemmap_populate() on its own schedule.

No objections.
vmemmap_populate_hugepages() seems like the best way to go for now given that
my patches will cause additional conflict with 5-level page tables.

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