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>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
> 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 | 42 +++++++++++++++++++++++++++++++-----------
> 1 file changed, 31 insertions(+), 11 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b65bc50..f4f4d57 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();
>
> @@ -1129,6 +1130,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>
> VM_BUG_ON_PAGE(PageActive(page), page);
>
> + nr_pages = 1 << compound_order(page);
> +
> + /*
> + * Accounted one page for THP for now. If THP gets swapped
> + * out in a whole, will account all tail pages later to
> + * avoid accounting tail pages twice.
> + */
> sc->nr_scanned++;
>
> if (unlikely(!page_evictable(page)))
> @@ -1250,7 +1258,7 @@ 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:
If the "Accessed bit" of a THP is set in the page table that maps it, it
will go PAGEREF_ACTIVATE path here. And the sc->nr_scanned should
increase 512 instead of 1. Otherwise sc->nr_activate may be larger than
sc->nr_scanned.
Best Regards,
Huang, Ying
