The LRU eviction algorithm needs to catch the first access to a loaded
page in order to call k_mem_paging_eviction_accessed() and move that
page to the tail of the queue. On ARM64 this is done with the MMU's
Access Flag: clearing AF causes a distinct fault on the next access.
On x86 there is no access-flag fault. The Accessed bit (PTE bit 5) is
set by hardware on access but never traps. The only way to force a
fault is to clear the Present bit, which already encodes the
"paged out" state — so a new state is needed:
PTE == 0 -> unmapped
P=0, A=1, upper=location -> paged out
P=0, G=1, upper=PFN -> LRU-tracked (new)
P=1 -> normally mapped
Bit G (Global, bit 8) is never set by Zephyr on x86 (CR4.PGE is not
used), so it is free to use as a private marker when P=0. No existing
PTE state needs to be displaced. This stays out of the way of the
KPTI path (which uses the PAT bit) and of the permission-backup bits
(IGNORED0..2) used for memory domain handling.
arch_page_info_get(addr, NULL, clear_accessed=true) is overloaded
under CONFIG_EVICTION_LRU to both query the prior flags and transition
the page to the LRU-tracked state via a new helper that updates all
domain ptables. arch_page_location_get() recognizes the tracked state
as paged-in so the core demand-paging code treats the page as resident.
The page fault handler intercepts LRU-tracking faults in-line before
k_mem_page_fault() dispatch: restore P, clear the tracking bit, and
call k_mem_paging_eviction_accessed() directly. This avoids the risk
of recursing through do_page_fault() with z_mm_lock held.
KPTI co-exists with demand paging but its PTE encoding is not yet
wired up to the LRU state, so tracking is gated on !X86_KPTI for now.
Fixes: #75132
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
b488151896