Re: Accounting problem of MIGRATE_ISOLATED freed page

[Minchan Kim]

On 06/21/2012 11:45 AM, KOSAKI Motohiro wrote:

> On Wed, Jun 20, 2012 at 9:55 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> On 06/21/2012 10:39 AM, KOSAKI Motohiro wrote:
>>
>>>>> number of isolate page block is almost always 0. then if we have such counter,
>>>>> we almost always can avoid zone->lock. Just idea.
>>>>
>>>> Yeb. I thought about it but unfortunately we can't have a counter for MIGRATE_ISOLATE.
>>>> Because we have to tweak in page free path for pages which are going to free later after we
>>>> mark pageblock type to MIGRATE_ISOLATE.
>>>
>>> I mean,
>>>
>>> if (nr_isolate_pageblock != 0)
>>>    free_pages -= nr_isolated_free_pages(); // your counting logic
>>>
>>> return __zone_watermark_ok(z, alloc_order, mark,
>>>                               classzone_idx, alloc_flags, free_pages);
>>>
>>>
>>> I don't think this logic affect your race. zone_watermark_ok() is already
>>> racy. then new little race is no big matter.
>>
>>
>> It seems my explanation wasn't enough. :(
>> I already understand your intention but we can't make nr_isolate_pageblock.
>> Because we should count two type of free pages.
> 
> I mean, move_freepages_block increment number of page *block*, not pages.
> number of free *pages* are counted by zone_watermark_ok_safe().
> 
> 
>> 1. Already freed page so they are already in buddy list.
>>   Of course, we can count it with return value of move_freepages_block(zone, page, MIGRATE_ISOLATE) easily.
>>
>> 2. Will be FREEed page by do_migrate_range.
>>   It's a _PROBLEM_. For it, we should tweak free path. No?
> 
> No.
> 
> 
>> If All of pages are PageLRU when hot-plug happens(ie, 2), nr_isolate_pagblock is zero and
>> zone_watermk_ok_safe can't do his role.
> 
> number of isolate pageblock don't depend on number of free pages. It's
> a concept of
> an attribute of PFN range.


It seems you mean is_migrate_isolate as a just flag, NOT nr_isolate_pageblock.
So do you mean this?

diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
index 3bdcab3..7f4d19c 100644
--- a/include/linux/page-isolation.h
+++ b/include/linux/page-isolation.h
@@ -1,6 +1,7 @@
 #ifndef __LINUX_PAGEISOLATION_H
 #define __LINUX_PAGEISOLATION_H
 
+extern bool is_migrate_isolate;
 /*
  * Changes migrate type in [start_pfn, end_pfn) to be MIGRATE_ISOLATE.
  * If specified range includes migrate types other than MOVABLE or CMA,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d2a515d..b997cb3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1756,6 +1756,27 @@ bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long ma
        if (z->percpu_drift_mark && free_pages < z->percpu_drift_mark)
                free_pages = zone_page_state_snapshot(z, NR_FREE_PAGES);
 
+#if defined CONFIG_CMA || CONFIG_MEMORY_HOTPLUG
+       if (unlikely(is_migrate_isolate)) {
+               unsigned long flags;
+               spin_lock_irqsave(&z->lock, flags);
+               for (order = MAX_ORDER - 1; order >= 0; order--) {
+                       struct free_area *area = &z->free_area[order];
+                       long count = 0;
+                       struct list_head *curr;
+
+                       list_for_each(curr, &area->free_list[MIGRATE_ISOLATE])
+                               count++;
+
+                       free_pages -= (count << order);
+                       if (free_pages < 0) {
+                               free_pages = 0;
+                               break;
+                       }
+               }
+               spin_unlock_irqrestore(&z->lock, flags);
+       }
+#endif
        return __zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags,
                                                                free_pages);
 }
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index c9f0477..212e526 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -19,6 +19,8 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
        return pfn_to_page(pfn + i);
 }
 
+bool is_migrate_isolate = false;
+
 /*
  * start_isolate_page_range() -- make page-allocation-type of range of pages
  * to be MIGRATE_ISOLATE.
@@ -43,6 +45,8 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
        BUG_ON((start_pfn) & (pageblock_nr_pages - 1));
        BUG_ON((end_pfn) & (pageblock_nr_pages - 1));
 
+       is_migrate_isolate = true;
+
        for (pfn = start_pfn;
             pfn < end_pfn;
             pfn += pageblock_nr_pages) {
@@ -59,6 +63,7 @@ undo:
             pfn += pageblock_nr_pages)
                unset_migratetype_isolate(pfn_to_page(pfn), migratetype);
 
+       is_migrate_isolate = false;
        return -EBUSY;
 }
 
@@ -80,6 +85,9 @@ int undo_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
                        continue;
                unset_migratetype_isolate(page, migratetype);
        }
+
+       is_migrate_isolate = false;
+
        return 0;
 }
 /*

It is still racy as you already mentioned and I don't think it's trivial.
Direct reclaim can't wake up kswapd forever by current fragile zone->all_unreclaimable.
So it's a livelock.
Then, do you want to fix this problem by your patch[1]?

It could solve the livelock by OOM kill if we apply your patch[1] but still doesn't wake up
kswapd although it's not critical. Okay. Then, please write down this problem in detail
in your patch's changelog and resend, please. 

[1] http://lkml.org/lkml/2012/6/14/74

-- 
Kind regards,
Minchan Kim
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