[PATCH v2 1/6] alpha: add missing page_table_check_pte_clear() to ptep_get_and_clear()

[Matt Turner]

Alpha's CONFIG_COMPACTION-gated ptep_get_and_clear() overrides the
generic version in include/linux/pgtable.h but omits its
page_table_check_pte_clear() call. With CONFIG_PAGE_TABLE_CHECK=y the
map count taken by set_ptes() is therefore never dropped when a PTE is
cleared through this path. zap_pte_range() clears PTEs this way, so the
first page freed after it hits the BUG_ON() in
__page_table_check_zero().

The call was instead placed in ptep_clear_flush() by commit dd5712f3379c
("alpha: fix user-space corruption during memory compaction"), where it
was harmless until alpha enabled page table check.

Add the missing call, and drop the now-duplicate one in
ptep_clear_flush(), which already goes through ptep_get_and_clear().

Fixes: e761b6fe4085 ("alpha: add ARCH_SUPPORTS_PAGE_TABLE_CHECK support")
Reviewed-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Tested-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Signed-off-by: Matt Turner <mattst88@xxxxxxxxx>
---
 arch/alpha/include/asm/pgtable.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/alpha/include/asm/pgtable.h b/arch/alpha/include/asm/pgtable.h
index 6ebdd90f3035..8a175e0c2b42 100644
--- a/arch/alpha/include/asm/pgtable.h
+++ b/arch/alpha/include/asm/pgtable.h
@@ -305,6 +305,7 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
 	pte_t pte = READ_ONCE(*ptep);
 
 	pte_clear(mm, address, ptep);
+	page_table_check_pte_clear(mm, address, pte);
 	return pte;
 }
 
@@ -316,7 +317,6 @@ static inline pte_t ptep_clear_flush(struct vm_area_struct *vma,
 	struct mm_struct *mm = vma->vm_mm;
 	pte_t pte = ptep_get_and_clear(mm, addr, ptep);
 
-	page_table_check_pte_clear(mm, addr, pte);
 	migrate_flush_tlb_page(vma, addr);
 	return pte;
 }

-- 
2.55.0