On 21/02/2019 21:06, Kirill A. Shutemov wrote:

On Thu, Feb 21, 2019 at 05:16:46PM +0000, Steven Price wrote:

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Note that in terms of the new page walking code, these new defines are
only used when walking a page table without a VMA (which isn't currently
done), so architectures which don't use p?d_large currently will work
fine with the generic versions. They only need to provide meaningful
definitions when switching to use the walk-without-a-VMA functionality.

How other architectures would know that they need to provide the helpers
to get walk-without-a-VMA functionality? This looks very fragile to me.

Yes, you've got a good point there. This would apply to the p?d_large
macros as well - any arch which (inadvertently) uses the generic version
is likely to be fragile/broken.

I think probably the best option here is to scrap the generic versions
altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which
would enable the new functionality to those arches that opt-in. Do you
think this would be less fragile?

These helpers are useful beyond pagewalker.

Can we actually do some grinding and make *all* archs to provide correct
helpers? Yes, it's tedious, but not that bad.

I think we could provide generic helpers for folded levels in
<asm-generic/pgtable-nop?d.h> and rest has to be provided by the arch.
Architectures that support only 2 level paging would need to provide
pgd_large(), with 3 -- pmd_large() and so on.

Fair enough, I'll have a go and hopefully people will be able to correct
it if I make any mistakes - I'm certainly not going to be able to test
all architectures myself.

Steve

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