x86/mm/pat: restore large ROX pages after fragmentation

x86/mm/pat: restore large ROX pages after fragmentation

Change of attributes of the pages may lead to fragmentation of direct
mapping over time and performance degradation when these pages contain
executable code.

With current code it's one way road: kernel tries to avoid splitting
large pages, but it doesn't restore them back even if page attributes
got compatible again.

Any change to the mapping may potentially allow to restore large page.

Add a hook to cpa_flush() path that will check if the pages in the range
that were just touched can be mapped at PMD level. If the collapse at the
PMD level succeeded, also attempt to collapse PUD level.

The collapse logic runs only when a set_memory_ method explicitly sets
CPA_COLLAPSE flag, for now this is only enabled in set_memory_rox().

CPUs don't like[1] to have to have TLB entries of different size for the
same memory, but looks like it's okay as long as these entries have
matching attributes[2]. Therefore it's critical to flush TLB before any
following changes to the mapping.

Note that we already allow for multiple TLB entries of different sizes
for the same memory now in split_large_page() path. It's not a new
situation.

set_memory_4k() provides a way to use 4k pages on purpose. Kernel must
not remap such pages as large. Re-use one of software PTE bits to
indicate such pages.

[1] See Erratum 383 of AMD Family 10h Processors
[2] https://lore.kernel.org/linux-mm/1da1b025-cabc-6f04-bde5-e50830d1ecf0@amd.com/

[rppt@kernel.org:
 * s/restore/collapse/
 * update formatting per peterz
 * use 'struct ptdesc' instead of 'struct page' for list of page tables to
   be freed
 * try to collapse PMD first and if it succeeds move on to PUD as peterz
   suggested
 * flush TLB twice: for changes done in the original CPA call and after
   collapsing of large pages
 * update commit message
]

Signed-off-by: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Co-developed-by: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
Signed-off-by: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250126074733.1384926-4-rppt@kernel.org