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Date: Mon, 27 May 2019 15:40:52 +0800
From: Yang Shi <yang.shi@...ux.alibaba.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: hannes@...xchg.org, mhocko@...e.com, mgorman@...hsingularity.net,
kirill.shutemov@...ux.intel.com, josef@...icpanda.com,
hughd@...gle.com, shakeelb@...gle.com, hdanton@...a.com,
akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [v6 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP
swapout
On 5/27/19 3:06 PM, Huang, Ying wrote:
> Yang Shi <yang.shi@...ux.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@...el.com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Cc: "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
>> Cc: Hugh Dickins <hughd@...gle.com>
>> Cc: Shakeel Butt <shakeelb@...gle.com>
>> Cc: Hillf Danton <hdanton@...a.com>
>> Signed-off-by: Yang Shi <yang.shi@...ux.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;
+ }
}
>
> Best Regards,
> Huang, Ying
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