Re: [PATCH v2] KVM: arm64: Try PMD block mappings if PUD mappings are not supported

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Punit,

Thank you for having a look!

On 9/11/20 9:34 AM, Punit Agrawal wrote:
> Hi Alexandru,
>
> Alexandru Elisei <alexandru.elisei@arm.com> writes:
>
>> When userspace uses hugetlbfs for the VM memory, user_mem_abort() tries to
>> use the same block size to map the faulting IPA in stage 2. If stage 2
>> cannot the same block mapping because the block size doesn't fit in the
>> memslot or the memslot is not properly aligned, user_mem_abort() will fall
>> back to a page mapping, regardless of the block size. We can do better for
>> PUD backed hugetlbfs by checking if a PMD block mapping is supported before
>> deciding to use a page.
> I think this was discussed in the past.
>
> I have a vague recollection of there being a problem if the user and
> stage 2 mappings go out of sync - can't recall the exact details.

I'm not sure what you mean by the two tables going out of sync. I'm looking at
Documentation/vm/unevictable-lru.rst and this is what it says regarding hugetlbfs:

"VMAs mapping hugetlbfs page are already effectively pinned into memory.  We
neither need nor want to mlock() these pages.  However, to preserve the prior
behavior of mlock() - before the unevictable/mlock changes - mlock_fixup() will
call make_pages_present() in the hugetlbfs VMA range to allocate the huge pages
and populate the ptes."

Please correct me if I'm wrong, but my interpretation is that once a hugetlbfs
page has been mapped in a process' address space, the only way to unmap it is via
munmap. If that's the case, the KVM mmu notifier should take care of unmapping
from stage 2 the entire memory range addressed by the hugetlbfs pages, right?

>
> Putting it out there in case anybody else on the thread can recall the
> details of the previous discussion (offlist).
>
> Though things may have changed and if it passes testing - then maybe I
> am mis-remembering. I'll take a closer look at the patch and shout out
> if I notice anything.

The test I ran was to boot a VM and run ltp (with printk's sprinkled in the host
kernel to see what page size and where it gets mapped/unmapped at stage 2). Do you
mind recommending other tests that I might run?

Thanks,
Alex

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