linux-kernel - [PATCH v2] mm,thp: recheck each page before collapsing file THP

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Message-ID: <20191018163754.3736610-1-songliubraving@fb.com>
Date:   Fri, 18 Oct 2019 09:37:54 -0700
From:   Song Liu <songliubraving@...com>
To:     <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <akpm@...ux-foundation.org>
CC:     <matthew.wilcox@...cle.com>, <kernel-team@...com>,
        <william.kucharski@...cle.com>, <kirill.shutemov@...ux.intel.com>,
        Song Liu <songliubraving@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: [PATCH v2] mm,thp: recheck each page before collapsing file THP

In collapse_file(), after locking the page, it is necessary to recheck
that the page is up-to-date. Add PageUptodate() check for both shmem THP
and file THP.

Current khugepaged should not try to collapse dirty file THP, because it
is limited to read only text. Add a PageDirty check and warning for file
THP. This is added after page_mapping() check, because if the page is
truncated, it might be dirty.

Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: William Kucharski <william.kucharski@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Song Liu <songliubraving@...com>

---
Changes v1 => v2:
Rearrange the checks per feedback from Johannes, Rik, and Kirill. 

---
 mm/khugepaged.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0a1b4b484ac5..40c549302d36 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1626,7 +1626,12 @@ static void collapse_file(struct mm_struct *mm,
 		 * without racing with truncate.
 		 */
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
-		VM_BUG_ON_PAGE(!PageUptodate(page), page);
+
+		/* double check the page is up to date */
+		if (unlikely(!PageUptodate(page))) {
+			result = SCAN_FAIL;
+			goto out_unlock;
+		}

 		/*
 		 * If file was truncated then extended, or hole-punched, before
@@ -1642,6 +1647,15 @@ static void collapse_file(struct mm_struct *mm,
 			goto out_unlock;
 		}

+		/*
+		 * khugepaged should not try to collapse dirty pages for
+		 * file THP. Show warning if this somehow happens.
+		 */
+		if (WARN_ON_ONCE(!is_shmem && PageDirty(page))) {
+			result = SCAN_FAIL;
+			goto out_unlock;
+		}
+
 		if (isolate_lru_page(page)) {
 			result = SCAN_DEL_PAGE_LRU;
 			goto out_unlock;
-- 
2.17.1