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

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Message-Id: <20200413100447.20073-6-kirill.shutemov@linux.intel.com>
Date:   Mon, 13 Apr 2020 13:04:46 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     akpm@...ux-foundation.org, Andrea Arcangeli <aarcange@...hat.com>
Cc:     Zi Yan <ziy@...dia.com>, Yang Shi <yang.shi@...ux.alibaba.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        William Kucharski <william.kucharski@...cle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCH 5/6] khugepaged: Allow to collapse a page shared across fork

The page can be included into collapse as long as it doesn't have extra
pins (from GUP or otherwise).

Logic to check the refcound is moved to a separate function.
Note that the function is ready to deal with compound pages. It's
preparation for the following patch.

VM_BUG_ON_PAGE() was removed from __collapse_huge_page_copy() as the
invariant it checks is no longer valid: the source can be mapped
multiple times now.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 mm/khugepaged.c | 41 ++++++++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 11 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e3e41c2768d8..f9864644c3b7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -529,6 +529,24 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte)
 	}
 }
 
+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);
+		dump_page(page, "Unexpected refcount");
+	}
+
+	return page_count(page) == expected_refcount;
+}
+
 static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 					unsigned long address,
 					pte_t *pte)
@@ -581,11 +599,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		}
 
 		/*
-		 * cannot use mapcount: can't collapse if there's a gup pin.
-		 * The page must only be referenced by the scanned process
-		 * and page swap cache.
+		 * Check if the page has any GUP (or other external) pins.
+		 *
+		 * The page table that maps the page has been already unlinked
+		 * from the page table tree and this process cannot get
+		 * an additinal pin on the page.
+		 *
+		 * New pins can come later if the page is shared across fork,
+		 * but not for the this process. It is fine. The other process
+		 * cannot write to the page, only trigger CoW.
 		 */
-		if (page_count(page) != 1 + PageSwapCache(page)) {
+		if (!is_refcount_suitable(page)) {
 			unlock_page(page);
 			result = SCAN_PAGE_COUNT;
 			goto out;
@@ -672,7 +696,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
 		} else {
 			src_page = pte_page(pteval);
 			copy_user_highpage(page, src_page, address, vma);
-			VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
 			release_pte_page(src_page);
 			/*
 			 * ptl mostly unnecessary, but preempt has to
@@ -1201,12 +1224,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 			goto out_unmap;
 		}
 
-		/*
-		 * cannot use mapcount: can't collapse if there's a gup pin.
-		 * The page must only be referenced by the scanned process
-		 * and page swap cache.
-		 */
-		if (page_count(page) != 1 + PageSwapCache(page)) {
+		/* Check if the page has any GUP (or other external) pins */
+		if (!is_refcount_suitable(page)) {
 			result = SCAN_PAGE_COUNT;
 			goto out_unmap;
 		}
-- 
2.26.0