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SubjectRe: [PATCH 10/10] mm: page_alloc: consolidate free page accounting
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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;
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