On Fri, Apr 24, 2026 at 07:51:44AM -0400, Peter Xu wrote: > On Fri, Apr 24, 2026 at 11:34:48AM +0100, Kiryl Shutsemau wrote: > > Both page_idle and the LRUs (legacy or MGLRU) track accesses on physical > > memory. We need visibility in the virtual address space domain. > > Yes they are, but ACCESS bit isn't.
A-bit is not a reliable signal for userspace working-set tracking because the kernel itself is a concurrent consumer. It is exactly why page_idle needs PG_young on top of the A-bit: PG_young is the "kernel ate the A-bit but the page was actually touched" escape hatch. And bringing PG_young into the picture puts us right back into physical-side tracking. > For migration, see e.g. remove_migration_pte() has: > > if (!softleaf_is_migration_young(entry)) > pte = pte_mkold(pte); remove_migration_pte() only propagates young-at-unmap. It does not cover the common case: A-bit cleared by reclaim before migration started. The concurrent-consumer problem is what breaks the signal, not the migration boundary. -- Kiryl Shutsemau / Kirill A. Shutemov
