linux-ext4 - [RFC 09/10] mm/khugepage.c: Warn if trying to scan devmap pmd

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Date: Thu, 11 Apr 2024 10:57:30 +1000
From: Alistair Popple <apopple@...dia.com>
To: linux-mm@...ck.org
Cc: david@...morbit.com,
	dan.j.williams@...el.com,
	jhubbard@...dia.com,
	rcampbell@...dia.com,
	willy@...radead.org,
	jgg@...dia.com,
	linux-fsdevel@...r.kernel.org,
	jack@...e.cz,
	djwong@...nel.org,
	hch@....de,
	david@...hat.com,
	ruansy.fnst@...itsu.com,
	nvdimm@...ts.linux.dev,
	linux-xfs@...r.kernel.org,
	linux-ext4@...r.kernel.org,
	jglisse@...hat.com,
	Alistair Popple <apopple@...dia.com>
Subject: [RFC 09/10] mm/khugepage.c: Warn if trying to scan devmap pmd

The only user of devmap PTEs is FS DAX, and khugepaged should not be
scanning these VMAs. This is checked by calling
hugepage_vma_check. Therefore khugepaged should never encounter a
devmap PTE. Warn if this occurs.

Signed-off-by: Alistair Popple <apopple@...dia.com>

---

Note this is a transitory patch to test the above assumption both at
runtime and during review. I will likely remove it as the whole thing
gets deleted when pXX_devmap is removed.
---
 mm/khugepaged.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 88433cc..b10db15 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -955,7 +955,7 @@ static int find_pmd_or_thp_or_none(struct mm_struct *mm,
 		return SCAN_PMD_NULL;
 	if (pmd_trans_huge(pmde))
 		return SCAN_PMD_MAPPED;
-	if (pmd_devmap(pmde))
+	if (WARN_ON_ONCE(pmd_devmap(pmde)))
 		return SCAN_PMD_NULL;
 	if (pmd_bad(pmde))
 		return SCAN_PMD_NULL;
-- 
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