[PATCH v2] mm/khugepaged: check compound_order() in collapse_pte_mapped_thp()

From: "Zach O'Keefe" <zokeefe@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, Yang Shi <shy828301@gmail.com>,
	 "Zach O'Keefe" <zokeefe@google.com>
Subject: [PATCH v2] mm/khugepaged: check compound_order() in collapse_pte_mapped_thp()
Date: Thu, 22 Sep 2022 15:27:31 -0700	[thread overview]
Message-ID: <20220922222731.1124481-1-zokeefe@google.com> (raw)

By the time we lock a page in collapse_pte_mapped_thp(), the page
mapped by the address pushed onto the slot's .pte_mapped_thp[] array
might have changed arbitrarily since we last looked at it.  We
revalidate that the page is still the head of a compound page, but we
don't revalidate if the compound page is of order HPAGE_PMD_ORDER before
applying rmap and page table updates.

Since the kernel now supports large folios of arbitrary order, and since
replacing page's pte mappings by a pmd mapping only makes sense for
compound pages of order HPAGE_PMD_ORDER, revalidate that the compound
order is indeed of order HPAGE_PMD_ORDER before proceeding.

Link: https://lore.kernel.org/linux-mm/CAHbLzkon+2ky8v9ywGcsTUgXM_B35jt5NThYqQKXW2YV_GUacw@mail.gmail.com/
Suggested-by: Yang Shi <shy828301@gmail.com>
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
 mm/khugepaged.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 57af2c841b41..40fd9f7b3ed3 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1399,6 +1399,9 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
 	if (!PageHead(hpage))
 		goto drop_hpage;
 
+	if (compound_order(hpage) != HPAGE_PMD_ORDER)
+		goto drop_hpage;
+
 	if (find_pmd_or_thp_or_none(mm, haddr, &pmd) != SCAN_SUCCEED)
 		goto drop_hpage;
 
-- 
2.37.3.998.g577e59143f-goog


