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

On 02/25/2017 10:03 PM, Dan Williams wrote:

[ adding kasan folks ]

On Wed, Feb 15, 2017 at 12:58 PM, Nicolai Stange [off-list ref] wrote:

quoted

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.



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