On 15 December 2016 at 15:39, Robert Richter [off-list ref] wrote:

I was going to do some measurements but my kernel crashes now with a
page fault in efi_rtc_probe():

[   21.663393] Unable to handle kernel paging request at virtual address 20251000
[   21.663396] pgd = ffff000009090000
[   21.663401] [20251000] *pgd=0000010ffff90003
[   21.663402] , *pud=0000010ffff90003
[   21.663404] , *pmd=0000000fdc030003
[   21.663405] , *pte=00e8832000250707

The sparsemem config requires the whole section to be initialized.
Your patches do not address this.

96000047 is a third level translation fault, and the PTE address has
RES0 bits set. I don't see how this is related to sparsemem, could you
explain?

On 14.12.16 09:11:47, Ard Biesheuvel wrote:

quoted

+config HOLES_IN_ZONE
+     def_bool y
+     depends on NUMA

This enables pfn_valid_within() for arm64 and causes the check for
each page of a section. The arm64 implementation of pfn_valid() is
already expensive (traversing memblock areas). Now, this is increased
by a factor of 2^18 for 4k page size (16384 for 64k). We need to
initialize the whole section to avoid that.

I know that. But if you want something for -stable, we should have
something that is correct first, and only then care about the
performance hit (if there is one)