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

["Huang\, Ying" <ying.huang@intel.com>]

Yang Shi <yang.shi@linux.alibaba.com> writes:

> On 5/27/19 3:06 PM, Huang, Ying wrote:
>> Yang Shi <yang.shi@linux.alibaba.com> 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@intel.com>
>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: Mel Gorman <mgorman@techsingularity.net>
>>> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
>>> Cc: Hugh Dickins <hughd@google.com>
>>> Cc: Shakeel Butt <shakeelb@google.com>
>>> Cc: Hillf Danton <hdanton@sina.com>
>>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>>> ---
>>> 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?
>
> It sounds not correct. If swapout fails it jumps to activate_locked
> too, it has to be handled in if (!add_to_swap(page)). The below fix
> should be good enough since only THP can reach here:
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 378edff..fff3937 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1294,8 +1294,15 @@ static unsigned long shrink_page_list(struct
> list_head *page_list,
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> count_vm_event(THP_SWPOUT_FALLBACK);
>  #endif
> -                                       if (!add_to_swap(page))
> +                                       if (!add_to_swap(page)) {
> +                                               /*
> +                                                * Minus tail pages
> and reset
> +                                                * nr_pages.
> +                                                */
> +                                               sc->nr_scanned -= 
> (nr_pages - 1);
> +                                               nr_pages = 1;
>                                                 goto activate_locked;
> +                                       }
>                                 }

I think you need to add similar logic for the first add_to_swap() in the
original code snippet.

>> 				if (!add_to_swap(page)) {
>> 					if (!PageTransHuge(page))

To reduce code duplication, I suggest to add another label to deal with
it.

activate_locked_split:
        if (nr_pages > 1) {
                sc->nr_scanned -= nr_pages - 1;
                nr_pages = 1;
        }

activate_locked:

And use "goto active_locked_split" if add_to_swap() failed.

Best Regards,
Huang, Ying