linux-kernel - Re: [PATCHv3, RESEND 0/8] thp/khugepaged improvements and CoW semantics

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Date:   Tue, 14 Apr 2020 20:47:18 -0600
From:   William Kucharski <william.kucharski@...cle.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Zi Yan <ziy@...dia.com>, Yang Shi <yang.shi@...ux.alibaba.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        linux-mm <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3, RESEND 0/8] thp/khugepaged improvements and CoW
 semantics 

Quick comments, mostly nits:

[PATCH 5/6] khugepaged: Allow to collapse a page shared across fork


> +static bool is_refcount_suitable(struct page *page)
> +{
> +	int expected_refcount, refcount;
> +
> +	refcount = page_count(page);
> +	expected_refcount = total_mapcount(page);
> +	if (PageSwapCache(page))
> +		expected_refcount += compound_nr(page);
> +
> +	if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) {
> +		pr_err("expected_refcount: %d, refcount: %d\n",
> +				expected_refcount, refcount);

I'd rather see this message reworded slightly as I prefer to know WHY a value
was unexpected directly from the error message:

		pr_err("expected_refcount (%d) > refcount (%d)\n",

[PATCHv3, RESEND 7/8] thp: Change CoW semantics for anon-THP

I really like the simplifications here.

[PATCHv3, RESEND 8/8] khugepaged: Introduce 'max_ptes_shared' tunable

As above, I'd like to see a tiny extra bit of specificity in the printf()s:

+static void collapse_max_ptes_shared()
+{

[ ... ]

+		if (check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		printf("Trigger CoW in %d of %d...",

		printf("Trigger CoW on page %d of %d...",

+				hpage_pmd_nr - max_ptes_shared - 1, hpage_pmd_nr);
+		fill_memory(p, 0, (hpage_pmd_nr - max_ptes_shared - 1) * page_size);
+		if (!check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		if (wait_for_scan("Do not collapse with max_ptes_shared exeeded", p))
+			fail("Timeout");
+		else if (!check_huge(p))
+			success("OK");
+		else
+			fail("Fail");
+
+		printf("Trigger CoW in %d of %d...",

		printf("Trigger CoW on page %d of %d...",

+				hpage_pmd_nr - max_ptes_shared, hpage_pmd_nr);
+		fill_memory(p, 0, (hpage_pmd_nr - max_ptes_shared) * page_size);
+		if (!check_huge(p))
+			success("OK");
+		else
+			fail("Fail");

Otherwise, for the khugepaged/THP series:

Reviewed-by: William Kucharski <william.kucharski@...cle.com>