On Tue, Aug 13, 2024 at 10:38 AM Yuanchu Xie <yuanchu@xxxxxxxxxx> wrote: > > When non-leaf pmd accessed bits are available, MGLRU page table walks > can clear the non-leaf pmd accessed bit and ignore the accessed bit on > the pte if it's on a different node, skipping a generation update as > well. If another scan occurs on the same node as said skipped pte. > the non-leaf pmd accessed bit might remain cleared and the pte accessed > bits won't be checked. While this is sufficient for reclaim-driven > aging, where the goal is to select a reasonably cold page, the access > can be missed when aging proactively for workingset estimation of a > node/memcg. > > In more detail, get_pfn_folio returns NULL if the folio's nid != node > under scanning, so the page table walk skips processing of said pte. Now > the pmd_young flag on this pmd is cleared, and if none of the pte's are > accessed before another scan occurs on the folio's node, the pmd_young > check fails and the pte accessed bit is skipped. > > Since force_scan disables various other optimizations, we check > force_scan to ignore the non-leaf pmd accessed bit. > > Signed-off-by: Yuanchu Xie <yuanchu@xxxxxxxxxx> Acked-by: Yu Zhao <yuzhao@xxxxxxxxxx>
