s390/pgtable: Add s390 support for page table check

s390/pgtable: Add s390 support for page table check

Add page table check hooks into routines that modify user page
tables.

Unlike other architectures s390 does not have means to distinguish
between kernel and user page table entries. Rely on the fact the
page table check infrastructure itself operates on non-init_mm
memory spaces only.

Use the provided mm_struct to verify that the memory space is not
init_mm (aka not the kernel memory space) indeed. That check is
supposed to be succeeded already (on some code paths even twice).

If the passed memory space by contrast is init_mm that would be an
unexpected semantical change in generic code, so do VM_BUG_ON() in
such case.

Unset _SEGMENT_ENTRY_READ bit to indicate that pmdp_invalidate()
was applied against a huge PMD and is going to be updated by
set_pmd_at() shortly. The hook pmd_user_accessible_page() should
skip such entries until that, otherwise the page table accounting
falls apart and BUG_ON() gets hit as result.

The invalidated huge PMD entry should not be confused with a PROT_NONE
entry as reported by pmd_protnone(), though the entry characteristics
exactly match: _SEGMENT_ENTRY_LARGE is set while _SEGMENT_ENTRY_READ is
unset. Since pmd_protnone() implementation depends on NUMA_BALANCING
configuration option, it should not be used in pmd_user_accessible_page()
check, which is expected to be CONFIG_NUMA_BALANCING-agnostic.

Nevertheless, an invalidated huge PMD is technically still pmd_protnone()
entry and it should not break other code paths once _SEGMENT_ENTRY_READ
is unset. As of now, all pmd_protnone() checks are done under page table
locks or exercise GUP-fast and HMM code paths, which are expected to be
safe against concurrent page table updates.

Alternative approach would be using the last remaining unused PMD entry
bit 0x800 to indicate that pmdp_invalidate() was called on a PMD. That
would allow avoiding collisions with pmd_protnone() handling code paths,
but saving the bit is more preferable way to go.

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
Co-developed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Link: https://lore.kernel.org/r/4db8a681205bd555298d62441cdcfca43317a35a.1772812343.git.agordeev@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>