Re: [v6 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Huang\, Ying]

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