[RFC patch] ioremap: don't set up huge I/O mappings when p4d/pud/pmd is zero

[Hanjun Guo]

From: Hanjun Guo <hanjun.guo at linaro.org>

When we using iounmap() to free the 4K mapping, it just clear the PTEs
but leave P4D/PUD/PMD unchanged, also will not free the memory of page
tables.

This will cause issues on ARM64 platform (not sure if other archs have
the same issue) for this case:

1. ioremap a 4K size, valid page table will build,
2. iounmap it, pte0 will set to 0;
3. ioremap the same address with 2M size, pgd/pmd is unchanged,
   then set the a new value for pmd;
4. pte0 is leaked;
5. CPU may meet exception because the old pmd is still in TLB,
   which will lead to kernel panic.

Fix it by skip setting up the huge I/O mappings when p4d/pud/pmd is
zero.

Reported-by: Lei Li <lious.lilei at hisilicon.com>
Signed-off-by: Hanjun Guo <hanjun.guo at linaro.org>
Cc: Toshi Kani <toshi.kani at hpe.com>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Xuefeng Wang <wxf.wang at hisilicon.com>
---

Not sure if this is the right direction, this patch has a obvious
side effect that a mapped address with 4K will not back to 2M.  I may
miss something and just wrong, so this is just a RFC version, comments
are welcomed.

 lib/ioremap.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/ioremap.c b/lib/ioremap.c
index b808a39..4e6f19a 100644
--- a/lib/ioremap.c
+++ b/lib/ioremap.c
@@ -89,7 +89,7 @@ static inline int ioremap_pmd_range(pud_t *pud, unsigned long addr,
 	do {
 		next = pmd_addr_end(addr, end);
 
-		if (ioremap_pmd_enabled() &&
+		if (ioremap_pmd_enabled() && pmd_none(*pmd) &&
 		    ((next - addr) == PMD_SIZE) &&
 		    IS_ALIGNED(phys_addr + addr, PMD_SIZE)) {
 			if (pmd_set_huge(pmd, phys_addr + addr, prot))
@@ -115,7 +115,7 @@ static inline int ioremap_pud_range(p4d_t *p4d, unsigned long addr,
 	do {
 		next = pud_addr_end(addr, end);
 
-		if (ioremap_pud_enabled() &&
+		if (ioremap_pud_enabled() && pud_none(*pud) &&
 		    ((next - addr) == PUD_SIZE) &&
 		    IS_ALIGNED(phys_addr + addr, PUD_SIZE)) {
 			if (pud_set_huge(pud, phys_addr + addr, prot))
@@ -141,7 +141,7 @@ static inline int ioremap_p4d_range(pgd_t *pgd, unsigned long addr,
 	do {
 		next = p4d_addr_end(addr, end);
 
-		if (ioremap_p4d_enabled() &&
+		if (ioremap_p4d_enabled() && p4d_none(*p4d) &&
 		    ((next - addr) == P4D_SIZE) &&
 		    IS_ALIGNED(phys_addr + addr, P4D_SIZE)) {
 			if (p4d_set_huge(p4d, phys_addr + addr, prot))
-- 
1.7.12.4