Re: [PATCH 09/13] mm/powerpc: Redefine pXd_huge() with pXd_leaf()

On Thu, Mar 14, 2024 at 08:45:34AM +0000, Christophe Leroy wrote:

Le 13/03/2024 à 22:47, peterx@redhat.com a écrit :

quoted

From: Peter Xu <peterx@redhat.com>

PowerPC book3s 4K mostly has the same definition on both, except pXd_huge()
constantly returns 0 for hash MMUs.  As Michael Ellerman pointed out [1],
it is safe to check _PAGE_PTE on hash MMUs, as the bit will never be set so
it will keep returning false.

As a reference, __p[mu]d_mkhuge() will trigger a BUG_ON trying to create
such huge mappings for 4K hash MMUs.  Meanwhile, the major powerpc hugetlb
pgtable walker __find_linux_pte() already used pXd_leaf() to check hugetlb
mappings.

The goal should be that we will have one API pXd_leaf() to detect all kinds
of huge mappings.  AFAICT we need to use the pXd_leaf() impl (rather than
pXd_huge() ones) to make sure ie. THPs on hash MMU will also return true.

All kinds of huge mappings ?

pXd_leaf() will detect only leaf mappings (like pXd_huge() ). There are 
also huge mappings through hugepd. On powerpc 8xx we have 8M huge pages 
and 512k huge pages. A PGD entry covers 4M so pgd_leaf() won't report 
those huge pages.

Ah yes, I should always mention this is in the context of leaf huge pages
only.  Are the examples you provided all fall into hugepd category?  If so
I can reword the commit message, as:

        As a reference, __p[mu]d_mkhuge() will trigger a BUG_ON trying to
        create such huge mappings for 4K hash MMUs.  Meanwhile, the major
        powerpc hugetlb pgtable walker __find_linux_pte() already used
        pXd_leaf() to check leaf hugetlb mappings.

        The goal should be that we will have one API pXd_leaf() to detect
        all kinds of huge mappings except hugepd.  AFAICT we need to use
        the pXd_leaf() impl (rather than pXd_huge() ones) to make sure
        ie. THPs on hash MMU will also return true.

Does this look good to you?

Thanks,

-- 
Peter Xu