Re: mmotm 2015-01-22-15-04: qemu failures due to 'mm: account pmd page tables to the process'

On Tue, 27 Jan 2015 08:24:28 -0800 Guenter Roeck [off-list ref] wrote:

quoted

__PAGETABLE_PMD_FOLDED is defined during <asm/pgtable.h> which is not
included into <linux/mm_types.h>. And we cannot include it here since
many of <asm/pgtables> needs <linux/mm_types.h> to define struct page.

I failed to come up with better solution rather than put nr_pmds into
mm_struct unconditionally.

One possible solution would be to expose number of page table levels
architecture has via Kconfig, but that's ugly and requires changes to
all architectures.

FWIW, I tried a number of approaches. Ultimately I gave up and concluded
that it has to be either this patch or, as you say here, we would have
to add something like PAGETABLE_PMD_FOLDED as a Kconfig option.

It's certainly a big mess.  Yes, I expect that moving
__PAGETABLE_PMD_FOLDED and probably PAGETABLE_LEVELS into Kconfig logic
would be a good fix.

Adding 8 bytes to the mm_struct (sometimes) isn't a huge issue, but
it does make the kernel just a little bit worse.

Has anyone taken a look at what the Kconfig approach would look like?

Possibly another fix for this would be to move mm_struct into its own
header file, or something along those lines?

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