Re: [PATCH V2] mm: Introduce GFP_PGTABLE

On Wed, Jan 16, 2019 at 02:47:16PM +0100, Christophe Leroy wrote:

Le 16/01/2019 à 14:18, Matthew Wilcox a écrit :

quoted

I disagree with your objective.  Making more code common is a great idea,
but this patch is too unambitious.  We should be heading towards one or
two page table allocation functions instead of having every architecture do
its own thing.

So start there.  Move the x86 function into common code and convert one
other architecture to use it too.

Are we talking about pte_alloc_one_kernel() and pte_alloc_one() ?

I'm not sure x86 function is the best common one, as it seems to allocate a
multiple of PAGE_SIZE only.

And that's the common case.  Most architectures use a single page for at
least one level of the pte/pmd/pud/p4d/pgd hierarchy.  Some use multiple
pages and some use a fraction of a page.

Some arches like powerpc use pagetables which are smaller than a page, for
instance powerpc 8xx uses 4k pagetables even with 16k pages, which means a
single page can be used by 4 pagetables.

Those can be added later.  Note I said "one or two", and that's what I
had in mind; I think we want one function that allocates just a page
and another that allocates a page fragment.  Then we can have a good
discussion about what method we use; s390 and ppc use different techniques
today and there's really no good reason for that.

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