ARM: 8115/1: LPAE: reduce damage caused by idmap to virtual memory layout

Konstantin Khlebnikov · Russell King · commit 811a2407a3cf · 2014-07-29T13:00:04.000+01:00
On LPAE, each level 1 (pgd) page table entry maps 1GiB, and the level 2 (pmd) entries map 2MiB. When the identity mapping is created on LPAE, the pgd pointers are copied from the swapper_pg_dir. If we find that we need to modify the contents of a pmd, we allocate a new empty pmd table and insert it into the appropriate 1GB slot, before then filling it with the identity mapping. However, if the 1GB slot covers the kernel lowmem mappings, we obliterate those mappings. When replacing a PMD, first copy the old PMD contents to the new PMD, so that we preserve the existing mappings, particularly the mappings of the kernel itself. [rewrote commit message and added code comment -- rmk] Fixes: ae2de10 ("ARM: LPAE: Add identity mapping support for the 3-level page table format") Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@samsung.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/mm/idmap.c b/arch/arm/mm/idmap.c
@@ -25,6 +25,13 @@ static void idmap_add_pmd(pud_t *pud, unsigned long addr, unsigned long end,
 			pr_warning("Failed to allocate identity pmd.\n");
 			return;
 		}
+		/*
+		 * Copy the original PMD to ensure that the PMD entries for
+		 * the kernel image are preserved.
+		 */
+		if (!pud_none(*pud))
+			memcpy(pmd, pmd_offset(pud, 0),
+			       PTRS_PER_PMD * sizeof(pmd_t));
 		pud_populate(&init_mm, pud, pmd);
 		pmd += pmd_index(addr);
 	} else
