On 15.12.16 16:07:26, Ard Biesheuvel wrote:

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

quoted

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?

When initializing the whole section it works. Maybe it uncovers
another bug. Did not yet start debugging this.

quoted

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)

I would prefer to check for a performance penalty *before* we put it
into stable. There is nor risk at all with the patch I am proposing.
See: https://lkml.org/lkml/2016/12/16/412

-Robert