Re: [PATCH RFC 04/13] mm/x86: Change pXd_huge() behavior to exclude swap entries

On Wed, Mar 06, 2024 at 06:41:38PM +0800, peterx@redhat.com wrote:

From: Peter Xu <peterx@redhat.com>

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/Y2WQ7I4LXh8iUIRd@intel.com/ (local)

  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/20240221125753.GQ13330@nvidia.com/ (local)

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/Y2LYXItKQyaJTv8j@intel.com/ (local)

Now we shouldn't need that.

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

Cc: Naoya Horiguchi <redacted>
Cc: Thomas Gleixner <redacted>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/x86/mm/hugetlbpage.c | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

I think this is the right thing to do, callers should be more directly
sensitive to swap entries not back into it indirectly from a helper
like this.

Jason