Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes: > Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after > swapped out"), THP can be swapped out in a whole. But, nr_reclaimed > and some other vm counters still get inc'ed by one even though a whole > THP (512 pages) gets swapped out. > > This doesn't make too much sense to memory reclaim. For example, direct > reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP > could fulfill it. But, if nr_reclaimed is not increased correctly, > direct reclaim may just waste time to reclaim more pages, > SWAP_CLUSTER_MAX * 512 pages in worst case. > > And, it may cause pgsteal_{kswapd|direct} is greater than > pgscan_{kswapd|direct}, like the below: > > pgsteal_kswapd 122933 > pgsteal_direct 26600225 > pgscan_kswapd 174153 > pgscan_direct 14678312 > > nr_reclaimed and nr_scanned must be fixed in parallel otherwise it would > break some page reclaim logic, e.g. > > vmpressure: this looks at the scanned/reclaimed ratio so it won't > change semantics as long as scanned & reclaimed are fixed in parallel. > > compaction/reclaim: compaction wants a certain number of physical pages > freed up before going back to compacting. > > kswapd priority raising: kswapd raises priority if we scan fewer pages > than the reclaim target (which itself is obviously expressed in order-0 > pages). As a result, kswapd can falsely raise its aggressiveness even > when it's making great progress. > > Other than nr_scanned and nr_reclaimed, some other counters, e.g. > pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail need to be fixed > too since they are user visible via cgroup, /proc/vmstat or trace > points, otherwise they would be underreported. > > When isolating pages from LRUs, nr_taken has been accounted in base > page, but nr_scanned and nr_skipped are still accounted in THP. It > doesn't make too much sense too since this may cause trace point > underreport the numbers as well. > > So accounting those counters in base page instead of accounting THP as > one page. > > nr_dirty, nr_unqueued_dirty, nr_congested and nr_writeback are used by > file cache, so they are not impacted by THP swap. > > This change may result in lower steal/scan ratio in some cases since > THP may get split during page reclaim, then a part of tail pages get > reclaimed instead of the whole 512 pages, but nr_scanned is accounted > by 512, particularly for direct reclaim. But, this should be not a > significant issue. > > Cc: "Huang, Ying" <ying.huang@xxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> > Cc: Hillf Danton <hdanton@xxxxxxxx> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> > --- > v6: Fixed the other double account issue introduced by v5 per Huang Ying > v5: Fixed sc->nr_scanned double accounting per Huang Ying > Added some comments to address the concern about premature OOM per Hillf Danton > v4: Fixed the comments from Johannes and Huang Ying > v3: Removed Shakeel's Reviewed-by since the patch has been changed significantly > Switched back to use compound_order per Matthew > Fixed more counters per Johannes > v2: Added Shakeel's Reviewed-by > Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski > > mm/vmscan.c | 47 +++++++++++++++++++++++++++++++++++------------ > 1 file changed, 35 insertions(+), 12 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index b65bc50..378edff 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1118,6 +1118,7 @@ static unsigned long shrink_page_list(struct list_head *page_list, > int may_enter_fs; > enum page_references references = PAGEREF_RECLAIM_CLEAN; > bool dirty, writeback; > + unsigned int nr_pages; > > cond_resched(); > > @@ -1129,7 +1130,10 @@ static unsigned long shrink_page_list(struct list_head *page_list, > > VM_BUG_ON_PAGE(PageActive(page), page); > > - sc->nr_scanned++; > + nr_pages = 1 << compound_order(page); > + > + /* Account the number of base pages even though THP */ > + sc->nr_scanned += nr_pages; > > if (unlikely(!page_evictable(page))) > goto activate_locked; > @@ -1250,7 +1254,7 @@ static unsigned long shrink_page_list(struct list_head *page_list, > case PAGEREF_ACTIVATE: > goto activate_locked; > case PAGEREF_KEEP: > - stat->nr_ref_keep++; > + stat->nr_ref_keep += nr_pages; > goto keep_locked; > case PAGEREF_RECLAIM: > case PAGEREF_RECLAIM_CLEAN: > @@ -1306,6 +1310,15 @@ static unsigned long shrink_page_list(struct list_head *page_list, > } > > /* > + * THP may get split above, need minus tail pages and update > + * nr_pages to avoid accounting tail pages twice. > + */ > + if ((nr_pages > 1) && !PageTransHuge(page)) { > + sc->nr_scanned -= (nr_pages - 1); > + nr_pages = 1; > + } After checking the code again, it appears there's another hole in the code. In the following code snippet. if (!add_to_swap(page)) { if (!PageTransHuge(page)) goto activate_locked; /* Fallback to swap normal pages */ if (split_huge_page_to_list(page, page_list)) goto activate_locked; #ifdef CONFIG_TRANSPARENT_HUGEPAGE count_vm_event(THP_SWPOUT_FALLBACK); #endif if (!add_to_swap(page)) goto activate_locked; } If the THP is split, but the first or the second add_to_swap() fails, we still need to deal with sc->nr_scanned and nr_pages. How about add a new label before "activate_locked" to deal with that? Best Regards, Huang, Ying