[v4 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

[v4 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

[Yang Shi]

The commit 9092c71bb724 ("mm: use sc->priority for slab shrink targets")
has broken up the relationship between sc->nr_scanned and slab pressure.
The sc->nr_scanned can't double slab pressure anymore.  So, it sounds no
sense to still keep sc->nr_scanned inc'ed.  Actually, it would prevent
from adding pressure on slab shrink since excessive sc->nr_scanned would
prevent from scan->priority raise.

The bonnie test doesn't show this would change the behavior of
slab shrinkers.

				w/		w/o
			  /sec    %CP      /sec      %CP
Sequential delete: 	3960.6    94.6    3997.6     96.2
Random delete: 		2518      63.8    2561.6     64.6

The slight increase of "/sec" without the patch would be caused by the
slight increase of CPU usage.

Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Michal Hocko <mhocko@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
---
v4: Added Johannes's ack

 mm/vmscan.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7acd0af..b65bc50 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1137,11 +1137,6 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		if (!sc->may_unmap && page_mapped(page))
 			goto keep_locked;
 
-		/* Double the slab pressure for mapped and swapcache pages */
-		if ((page_mapped(page) || PageSwapCache(page)) &&
-		    !(PageAnon(page) && !PageSwapBacked(page)))
-			sc->nr_scanned++;
-
 		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
 			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
 
-- 
1.8.3.1

[v4 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Yang Shi]

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>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
---
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 | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b65bc50..1b35a7a 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 +1130,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 		VM_BUG_ON_PAGE(PageActive(page), page);
 
-		sc->nr_scanned++;
+		/* Account the number of base pages evne though THP */
+		nr_pages = 1 << compound_order(page);
+		sc->nr_scanned += nr_pages;
 
 		if (unlikely(!page_evictable(page)))
 			goto activate_locked;
@@ -1250,7 +1253,7 @@ 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:
@@ -1315,7 +1318,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			if (unlikely(PageTransHuge(page)))
 				flags |= TTU_SPLIT_HUGE_PMD;
 			if (!try_to_unmap(page, flags)) {
-				stat->nr_unmap_fail++;
+				stat->nr_unmap_fail += nr_pages;
 				goto activate_locked;
 			}
 		}
@@ -1442,7 +1445,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 		unlock_page(page);
 free_it:
-		nr_reclaimed++;
+		/*
+		 * THP may get swapped out in a whole, need account
+		 * all base pages.
+		 */
+		nr_reclaimed += (1 << compound_order(page));
 
 		/*
 		 * Is there need to periodically free_page_list? It would
@@ -1464,7 +1471,6 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		if (!PageMlocked(page)) {
 			int type = page_is_file_cache(page);
 			SetPageActive(page);
-			pgactivate++;
 			stat->nr_activate[type] += hpage_nr_pages(page);
 			count_memcg_page_event(page, PGACTIVATE);
 		}
@@ -1475,6 +1481,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
 	}
 
+	pgactivate = stat->nr_activate[0] + stat->nr_activate[1];
+
 	mem_cgroup_uncharge_list(&free_pages);
 	try_to_unmap_flush();
 	free_unref_page_list(&free_pages);
@@ -1642,14 +1650,14 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 	unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
 	unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
 	unsigned long skipped = 0;
-	unsigned long scan, total_scan, nr_pages;
+	unsigned long scan, total_scan;
+	unsigned long nr_pages;
 	LIST_HEAD(pages_skipped);
 	isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
 
+	total_scan = 0;
 	scan = 0;
-	for (total_scan = 0;
-	     scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src);
-	     total_scan++) {
+	while (scan < nr_to_scan && !list_empty(src)) {
 		struct page *page;
 
 		page = lru_to_page(src);
@@ -1657,9 +1665,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 
 		VM_BUG_ON_PAGE(!PageLRU(page), page);
 
+		nr_pages = 1 << compound_order(page);
+		total_scan += nr_pages;
+
 		if (page_zonenum(page) > sc->reclaim_idx) {
 			list_move(&page->lru, &pages_skipped);
-			nr_skipped[page_zonenum(page)]++;
+			nr_skipped[page_zonenum(page)] += nr_pages;
 			continue;
 		}
 
@@ -1669,10 +1680,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 		 * ineligible pages.  This causes the VM to not reclaim any
 		 * pages, triggering a premature OOM.
 		 */
-		scan++;
+		scan += nr_pages;
 		switch (__isolate_lru_page(page, mode)) {
 		case 0:
-			nr_pages = hpage_nr_pages(page);
 			nr_taken += nr_pages;
 			nr_zone_taken[page_zonenum(page)] += nr_pages;
 			list_move(&page->lru, dst);
-- 
1.8.3.1

Re: [v4 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Huang, Ying]

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>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> ---
> 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 | 34 ++++++++++++++++++++++------------
>  1 file changed, 22 insertions(+), 12 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b65bc50..1b35a7a 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 +1130,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  
>  		VM_BUG_ON_PAGE(PageActive(page), page);
>  
> -		sc->nr_scanned++;
> +		/* Account the number of base pages evne though THP */

s/evne/even/

> +		nr_pages = 1 << compound_order(page);
> +		sc->nr_scanned += nr_pages;
>  
>  		if (unlikely(!page_evictable(page)))
>  			goto activate_locked;
> @@ -1250,7 +1253,7 @@ 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 THP is split, you need

        sc->nr_scanned -= nr_pages - 1;

Otherwise the tail pages will be counted twice.

> @@ -1315,7 +1318,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  			if (unlikely(PageTransHuge(page)))
>  				flags |= TTU_SPLIT_HUGE_PMD;
>  			if (!try_to_unmap(page, flags)) {
> -				stat->nr_unmap_fail++;
> +				stat->nr_unmap_fail += nr_pages;
>  				goto activate_locked;
>  			}
>  		}
> @@ -1442,7 +1445,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  
>  		unlock_page(page);
>  free_it:
> -		nr_reclaimed++;
> +		/*
> +		 * THP may get swapped out in a whole, need account
> +		 * all base pages.
> +		 */
> +		nr_reclaimed += (1 << compound_order(page));

Best Regards,
Huang, Ying

Re: [v4 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Yang Shi]

On 5/23/19 8:52 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>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> ---
>> 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 | 34 ++++++++++++++++++++++------------
>>   1 file changed, 22 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index b65bc50..1b35a7a 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 +1130,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>   
>>   		VM_BUG_ON_PAGE(PageActive(page), page);
>>   
>> -		sc->nr_scanned++;
>> +		/* Account the number of base pages evne though THP */
> s/evne/even/
>
>> +		nr_pages = 1 << compound_order(page);
>> +		sc->nr_scanned += nr_pages;
>>   
>>   		if (unlikely(!page_evictable(page)))
>>   			goto activate_locked;
>> @@ -1250,7 +1253,7 @@ 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 THP is split, you need
>
>          sc->nr_scanned -= nr_pages - 1;
>
> Otherwise the tail pages will be counted twice.

Yes, it looks so.

>
>> @@ -1315,7 +1318,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>   			if (unlikely(PageTransHuge(page)))
>>   				flags |= TTU_SPLIT_HUGE_PMD;
>>   			if (!try_to_unmap(page, flags)) {
>> -				stat->nr_unmap_fail++;
>> +				stat->nr_unmap_fail += nr_pages;
>>   				goto activate_locked;
>>   			}
>>   		}
>> @@ -1442,7 +1445,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>   
>>   		unlock_page(page);
>>   free_it:
>> -		nr_reclaimed++;
>> +		/*
>> +		 * THP may get swapped out in a whole, need account
>> +		 * all base pages.
>> +		 */
>> +		nr_reclaimed += (1 << compound_order(page));
> Best Regards,
> Huang, Ying

Re: [v4 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Yang Shi]

On 5/23/19 11:51 PM, Hillf Danton wrote:
> On Thu, 23 May 2019 10:27:38 +0800 Yang Shi wrote:
>> @ -1642,14 +1650,14 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>>   	unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
>>   	unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
>>   	unsigned long skipped = 0;
>> -	unsigned long scan, total_scan, nr_pages;
>> +	unsigned long scan, total_scan;
>> +	unsigned long nr_pages;
> Change for no earn:)

Aha, yes.

>
>>   	LIST_HEAD(pages_skipped);
>>   	isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
>>   
>> +	total_scan = 0;
>>   	scan = 0;
>> -	for (total_scan = 0;
>> -	     scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src);
>> -	     total_scan++) {
>> +	while (scan < nr_to_scan && !list_empty(src)) {
>>   		struct page *page;
> AFAICS scan currently prevents us from looping for ever, while nr_taken bails
> us out once we get what's expected, so I doubt it makes much sense to cut
> nr_taken off.

It is because "scan < nr_to_scan && nr_taken >= nr_to_scan" is 
impossible now with the units fixed.

>>   
>>   		page = lru_to_page(src);
>> @@ -1657,9 +1665,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>>   
>>   		VM_BUG_ON_PAGE(!PageLRU(page), page);
>>   
>> +		nr_pages = 1 << compound_order(page);
>> +		total_scan += nr_pages;
>> +
>>   		if (page_zonenum(page) > sc->reclaim_idx) {
>>   			list_move(&page->lru, &pages_skipped);
>> -			nr_skipped[page_zonenum(page)]++;
>> +			nr_skipped[page_zonenum(page)] += nr_pages;
>>   			continue;
>>   		}
>>   
>> @@ -1669,10 +1680,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>>   		 * ineligible pages.  This causes the VM to not reclaim any
>>   		 * pages, triggering a premature OOM.
>>   		 */
>> -		scan++;
>> +		scan += nr_pages;
> The comment looks to defy the change if we fail to add a huge page to
> the dst list; otherwise nr_taken knows how to do the right thing. What
> I prefer is to let scan to do one thing a time.

I don't get your point. Do you mean the comment "Do not count skipped 
pages because that makes the function return with no isolated pages if 
the LRU mostly contains ineligible pages."? I'm supposed the comment is 
used to explain why not count skipped page.

>
>>   		switch (__isolate_lru_page(page, mode)) {
>>   		case 0:
>> -			nr_pages = hpage_nr_pages(page);
>>   			nr_taken += nr_pages;
>>   			nr_zone_taken[page_zonenum(page)] += nr_pages;
>>   			list_move(&page->lru, dst);
>> -- 
>> 1.8.3.1
>>
> Best Regards
> Hillf