[PATCH] highmem: fix highmem for xtensa

[Max Filippov <jcmvbkbc@gmail.com>]

Fixmap on xtensa grows upwards, i.e. bigger fixmap entry index
corresponds to a higher virtual address. This was lost in highmem
generalization resulting in the following runtime warnings:

 WARNING: CPU: 0 PID: 18 at mm/highmem.c:494 kunmap_local_indexed+0x45/0x54
 Modules linked in:
 CPU: 0 PID: 18 Comm: kworker/u2:0 Not tainted 5.10.0-rc3-next-20201113 #1
 Call Trace:
   __warn+0x8f/0xc8
   warn_slowpath_fmt+0x35/0x70
   kunmap_local_indexed+0x45/0x54
   handle_mm_fault+0x325/0xbe0
   __get_user_pages.part.61+0x131/0x22c
   __get_user_pages+0x44/0x60
   __get_user_pages_remote+0xe8/0x290
   get_user_pages_remote+0x24/0x40
   get_arg_page+0x50/0x78
   copy_string_kernel+0x5c/0x120
   kernel_execve+0x76/0xc8
   call_usermodehelper_exec_async+0xc8/0x10c
   ret_from_kernel_thread+0xc/0x18

Fix it by adding __ARCH_HAS_POSITIVE_FIXMAP macro and implementing
vaddr_in_fixmap and fixmap_pte primitives differently depending on
whether it is defined or not.

Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 629ed3f7dad2 ("xtensa/mm/highmem: Switch to generic kmap atomic")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
---
 arch/xtensa/include/asm/fixmap.h |  2 ++
 mm/highmem.c                     | 29 ++++++++++++++++++++++++-----
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/arch/xtensa/include/asm/fixmap.h b/arch/xtensa/include/asm/fixmap.h
index 92049b61c351..66787b5f13d6 100644
--- a/arch/xtensa/include/asm/fixmap.h
+++ b/arch/xtensa/include/asm/fixmap.h
@@ -51,6 +51,8 @@ enum fixed_addresses {
 #define __fix_to_virt(x)	(FIXADDR_START + ((x) << PAGE_SHIFT))
 #define __virt_to_fix(x)	(((x) - FIXADDR_START) >> PAGE_SHIFT)
 
+#define __ARCH_HAS_POSITIVE_FIXMAP
+
 #ifndef __ASSEMBLY__
 /*
  * 'index to address' translation. If anyone tries to use the idx
diff --git a/mm/highmem.c b/mm/highmem.c
index 54bd233846c9..af27ed8d6a97 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -434,6 +434,26 @@ static inline void kmap_high_unmap_local(unsigned long vaddr)
 #endif
 }
 
+static inline bool vaddr_in_fixmap(unsigned long addr)
+{
+#ifdef __ARCH_HAS_POSITIVE_FIXMAP
+	return addr <= __fix_to_virt(FIX_KMAP_END) &&
+		addr >= __fix_to_virt(FIX_KMAP_BEGIN);
+#else
+	return addr >= __fix_to_virt(FIX_KMAP_END) &&
+		addr <= __fix_to_virt(FIX_KMAP_BEGIN);
+#endif
+}
+
+static pte_t *fixmap_pte(pte_t *kmap_pte, int idx)
+{
+#ifdef __ARCH_HAS_POSITIVE_FIXMAP
+	return kmap_pte + idx;
+#else
+	return kmap_pte - idx;
+#endif
+}
+
 static inline int kmap_local_calc_idx(int idx)
 {
 	return idx + KM_MAX_IDX * smp_processor_id();
@@ -457,9 +477,9 @@ void *__kmap_local_pfn_prot(unsigned long pfn, pgprot_t prot)
 	preempt_disable();
 	idx = arch_kmap_local_map_idx(kmap_local_idx_push(), pfn);
 	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
-	BUG_ON(!pte_none(*(kmap_pte - idx)));
+	BUG_ON(!pte_none(*(fixmap_pte(kmap_pte, idx))));
 	pteval = pfn_pte(pfn, prot);
-	set_pte_at(&init_mm, vaddr, kmap_pte - idx, pteval);
+	set_pte_at(&init_mm, vaddr, fixmap_pte(kmap_pte, idx), pteval);
 	arch_kmap_local_post_map(vaddr, pteval);
 	preempt_enable();
 
@@ -489,8 +509,7 @@ void kunmap_local_indexed(void *vaddr)
 	pte_t *kmap_pte = kmap_get_pte();
 	int idx;
 
-	if (addr < __fix_to_virt(FIX_KMAP_END) ||
-	    addr > __fix_to_virt(FIX_KMAP_BEGIN)) {
+	if (!vaddr_in_fixmap(addr)) {
 		WARN_ON_ONCE(addr < PAGE_OFFSET);
 
 		/* Handle mappings which were obtained by kmap_high_get() */
@@ -503,7 +522,7 @@ void kunmap_local_indexed(void *vaddr)
 	WARN_ON_ONCE(addr != __fix_to_virt(FIX_KMAP_BEGIN + idx));
 
 	arch_kmap_local_pre_unmap(addr);
-	pte_clear(&init_mm, addr, kmap_pte - idx);
+	pte_clear(&init_mm, addr, fixmap_pte(kmap_pte, idx));
 	arch_kmap_local_post_unmap(addr);
 	kmap_local_idx_pop();
 	preempt_enable();
-- 
2.20.1