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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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