mm/x86: change pXd_huge() behavior to exclude swap entries

This patch partly reverts below commits:

3a194f3f8ad0 ("mm/hugetlb: make pud_huge() and follow_huge_pud() aware of non-present pud entry")
cbef8478bee5 ("mm/hugetlb: pmd_huge() returns true for non-present hugepage")

Right now, pXd_huge() definition across kernel is unclear. We have two
groups that think differently on swap entries:

  - x86/sparc:     Allow pXd_huge() to accept swap entries
  - all the rest:  Doesn't allow pXd_huge() to accept swap entries

This is so confusing.  Since the sparc helpers seem to be added in 2016,
which is after x86's (2015), so sparc could have followed a trend.  x86
proposed such swap handling in 2015 to resolve hugetlb swap entries hit in
GUP, but now GUP guards swap entries with !pXd_present() in all layers so
we should be safe.

We should define this API properly, one way or another, rather than keep
them defined differently across archs.

Gut feeling tells me that pXd_huge() shouldn't include swap entries, and it
turns out that I am not the only one thinking so, the question was raised
when the current pmd_huge() for x86 was proposed by Ville Syrjälä:

https://lore.kernel.org/all/[email protected]/

  I might also be missing something obvious, but why is it even necessary
  to treat PRESENT==0+PSE==0 as a huge entry?

It is also questioned when Jason Gunthorpe reviewed the other patchset on
swap entry handlings:

https://lore.kernel.org/all/[email protected]/

Revert its meaning back to original.  It shouldn't have any functional
change as we should be ready with guards on !pXd_present() explicitly
everywhere.

Note that I also dropped the "#if CONFIG_PGTABLE_LEVELS > 2", it was there
probably because it was breaking things when 3a194f3f8ad0 was proposed,
according to the report here:

https://lore.kernel.org/all/[email protected]/

Now we shouldn't need that.

Instead of reverting to _PAGE_PSE raw check, leverage pXd_leaf().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Andreas Larsson <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Bjorn Andersson <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Konrad Dybcio <[email protected]>
Cc: Krzysztof Kozlowski <[email protected]>
Cc: Lucas Stach <[email protected]>
Cc: Mark Salter <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: "Naveen N. Rao" <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Russell King <[email protected]>
Cc: Shawn Guo <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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