On Mon, Apr 8, 2024 at 11:52 PM Huang, Ying ying.huang@intel.com wrote:
Yuanchu Xie yuanchu@google.com writes:
When non-leaf pmd accessed bits are available, MGLRU page table walks can clear the accessed bit and promptly ignore the accessed bit on the pte because it's on a different node, so the walk does not update the generation of said page. When the next scan comes around on the right node, 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 measuring the working set size of a node/memcg.
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@google.com
mm/vmscan.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c index 4f9c854ce6cc..1a7c7d537db6 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3522,7 +3522,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
walk->mm_stats[MM_NONLEAF_TOTAL]++;
if (should_clear_pmd_young()) {
if (!walk->force_scan && should_clear_pmd_young()) { if (!pmd_young(val)) continue;Sorry, I don't understand why we need this. If !pmd_young(val), we don't need to update the generation. If pmd_young(val), the bloom filter will be ignored if force_scan == true. Or do I miss something?
If !pmd_young(val), we still might need to update the generation.
The get_pfn_folio function returns NULL if the folio's nid != node under scanning, so the pte accessed bit does not get cleared and the generation is not updated. Now the pmd_young flag of this pmd is cleared, and if none of the pte's are accessed before another round of scanning occurs on the folio's node, the pmd_young check fails and the pte accessed bit is skipped.
This is fine for kswapd but can introduce inaccuracies when scanning proactively for workingset estimation.
Thanks, Yuanchu