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Date: Tue, 9 Apr 2024 17:31:08 +0800
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Subject: Re: [PATCH 10/10] mm: page_alloc: consolidate free page accounting
To: Johannes Weiner <hannes@cmpxchg.org>, Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
 Mel Gorman <mgorman@techsingularity.net>, Zi Yan <ziy@nvidia.com>,
 "Huang, Ying" <ying.huang@intel.com>, David Hildenbrand <david@redhat.com>,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20240320180429.678181-1-hannes@cmpxchg.org>
 <20240320180429.678181-11-hannes@cmpxchg.org>
 <7b3b7f2e-7109-4e72-b1cf-259cb56f3629@linux.alibaba.com>
 <ecb88320-9990-49e1-a58a-e8fc85b1da3f@suse.cz>
 <20240408142340.GA1057805@cmpxchg.org>
From: Baolin Wang <baolin.wang@linux.alibaba.com>
In-Reply-To: <20240408142340.GA1057805@cmpxchg.org>
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On 2024/4/8 22:23, Johannes Weiner wrote:
> On Mon, Apr 08, 2024 at 09:38:20AM +0200, Vlastimil Babka wrote:
>> On 4/7/24 12:19 PM, Baolin Wang wrote:
>>> On 2024/3/21 02:02, Johannes Weiner wrote:
>>>>    
>>>> +	account_freepages(page, zone, 1 << order, migratetype);
>>>> +
>>>>    	while (order < MAX_PAGE_ORDER) {
>>>> -		if (compaction_capture(capc, page, order, migratetype)) {
>>>> -			__mod_zone_freepage_state(zone, -(1 << order),
>>>> -								migratetype);
>>>> +		int buddy_mt = migratetype;
>>>> +
>>>> +		if (compaction_capture(capc, page, order, migratetype))
>>>>    			return;
>>>> -		}
>>>
>>> IIUC, if the released page is captured by compaction, then the
>>> statistics for free pages should be correspondingly decreased,
>>> otherwise, there will be a slight regression for my thpcompact benchmark.
>>>
>>> thpcompact Percentage Faults Huge
>>>                             k6.9-rc2-base        base + patch10 + 2 fixes	
>>> Percentage huge-1        78.18 (   0.00%)       71.92 (  -8.01%)
>>> Percentage huge-3        86.70 (   0.00%)       86.07 (  -0.73%)
>>> Percentage huge-5        90.26 (   0.00%)       78.02 ( -13.57%)
>>> Percentage huge-7        92.34 (   0.00%)       78.67 ( -14.81%)
>>> Percentage huge-12       91.18 (   0.00%)       81.04 ( -11.12%)
>>> Percentage huge-18       89.00 (   0.00%)       79.57 ( -10.60%)
>>> Percentage huge-24       90.52 (   0.00%)       80.07 ( -11.54%)
>>> Percentage huge-30       94.44 (   0.00%)       96.28 (   1.95%)
>>> Percentage huge-32       93.09 (   0.00%)       99.39 (   6.77%)
>>>
>>> I add below fix based on your fix 2, then the thpcompact Percentage
>>> looks good. How do you think for the fix?
>>
>> Yeah another well spotted, thanks. "slight regression" is an understatement,
>> this affects not just a "statistics" but very important counter
>> NR_FREE_PAGES which IIUC would eventually become larger than reality, make
>> the watermark checks false positive and result in depleted reserves etc etc.
>> Actually wondering why we're not seeing -next failures already (or maybe I
>> just haven't noticed).
> 
> Good catch indeed.
> 
> Trying to understand why I didn't notice this during testing, and I
> think it's because I had order-10 pageblocks in my config. There is
> this in compaction_capture():
> 
> 	if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
> 		return false;
> 
> Most compaction is for order-9 THPs on movable blocks, so I didn't get
> much capturing in practice in order for that leak to be noticable.

This makes me wonder why not use 'cc->migratetype' for migratetype 
comparison, so that low-order (like mTHP) compaction can directly get 
the released pages, which could avoid some compaction scans without 
mixing the migratetype?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2facf844ef84..7a64020f8222 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -622,7 +622,7 @@ compaction_capture(struct capture_control *capc, 
struct page *page,
          * and vice-versa but no more than normal fallback logic which can
          * have trouble finding a high-order free page.
          */
-       if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
+       if (order < pageblock_order && capc->cc->migratetype != migratetype)
                 return false;

         capc->page = page;