[PATCHv2 0/2] Add IO mapping space reused support

[PATCHv2 0/2] Add IO mapping space reused support

[Richard Lee]

Changes in V2:
 - Uses the atomic_t instead.
 - Revises some comment message.


Richard Lee (2):
  mm/vmalloc: Add IO mapping space reused interface support.
  ARM: ioremap: Add IO mapping space reused support.

 arch/arm/mm/ioremap.c   |  6 ++++
 include/linux/vmalloc.h |  5 +++
 mm/vmalloc.c            | 82 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 93 insertions(+)

-- 
1.8.4

[PATCHv2 1/2] mm/vmalloc: Add IO mapping space reused interface support.

[Richard Lee]

For the IO mapping, the same physical address space maybe
mapped more than one time, for example, in some SoCs:
  - 0x20001000 ~ 0x20001400 --> 1KB for Dev1
  - 0x20001400 ~ 0x20001800 --> 1KB for Dev2
  and the page size is 4KB.

Then both Dev1 and Dev2 will do ioremap operations, and the IO
vmalloc area's virtual address will be aligned down to 4KB, and
the size will be aligned up to 4KB. That's to say, only one
4KB size's vmalloc area could contain Dev1 and Dev2 IO mapping area
at the same time.

For this case, we can ioremap only one time, and the later ioremap
operation will just return the exist vmalloc area.

Signed-off-by: Richard Lee <superlibj8301@gmail.com>
---
 include/linux/vmalloc.h |  5 +++
 mm/vmalloc.c            | 82 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 87 insertions(+)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 4b8a891..a53b70f 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -1,6 +1,7 @@
 #ifndef _LINUX_VMALLOC_H
 #define _LINUX_VMALLOC_H
 
+#include <linux/atomic.h>
 #include <linux/spinlock.h>
 #include <linux/init.h>
 #include <linux/list.h>
@@ -34,6 +35,7 @@ struct vm_struct {
 	struct page		**pages;
 	unsigned int		nr_pages;
 	phys_addr_t		phys_addr;
+	atomic_t		used;
 	const void		*caller;
 };
 
@@ -100,6 +102,9 @@ static inline size_t get_vm_area_size(const struct vm_struct *area)
 	return area->size - PAGE_SIZE;
 }
 
+extern struct vm_struct *find_vm_area_paddr(phys_addr_t paddr, size_t size,
+				     unsigned long *offset,
+				     unsigned long flags);
 extern struct vm_struct *get_vm_area(unsigned long size, unsigned long flags);
 extern struct vm_struct *get_vm_area_caller(unsigned long size,
 					unsigned long flags, const void *caller);
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index bf233b2..cf0093c 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1293,6 +1293,7 @@ static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
 	vm->addr = (void *)va->va_start;
 	vm->size = va->va_end - va->va_start;
 	vm->caller = caller;
+	atomic_set(&vm->used, 1);
 	va->vm = vm;
 	va->flags |= VM_VM_AREA;
 	spin_unlock(&vmap_area_lock);
@@ -1383,6 +1384,84 @@ struct vm_struct *get_vm_area_caller(unsigned long size, unsigned long flags,
 				  NUMA_NO_NODE, GFP_KERNEL, caller);
 }
 
+static int vm_area_used_inc(struct vm_struct *area)
+{
+	if (!(area->flags & VM_IOREMAP))
+		return -EINVAL;
+
+	atomic_add(1, &area->used);
+
+	return atomic_read(&va->vm->used);
+}
+
+static int vm_area_used_dec(const void *addr)
+{
+	struct vmap_area *va;
+
+	va = find_vmap_area((unsigned long)addr);
+	if (!va || !(va->flags & VM_VM_AREA))
+		return 0;
+
+	if (!(va->vm->flags & VM_IOREMAP))
+		return 0;
+
+	atomic_sub(1, &va->vm->used);
+
+	return atomic_read(&va->vm->used);
+}
+
+/**
+ *	find_vm_area_paddr  -  find a continuous kernel virtual area using the
+ *			physical addreess.
+ *	@paddr:		base physical address
+ *	@size:		size of the physical area range
+ *	@offset:	the start offset of the vm area
+ *	@flags:		%VM_IOREMAP for I/O mappings
+ *
+ *	Search for the kernel VM area, whoes physical address starting at
+ *	@paddr, and if the exsit VM area's size is large enough, then return
+ *	it with increasing the 'used' counter, or return NULL.
+ */
+struct vm_struct *find_vm_area_paddr(phys_addr_t paddr, size_t size,
+				     unsigned long *offset,
+				     unsigned long flags)
+{
+	struct vmap_area *va;
+	int off;
+
+	if (!(flags & VM_IOREMAP))
+		return NULL;
+
+	size = PAGE_ALIGN((paddr & ~PAGE_MASK) + size);
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(va, &vmap_area_list, list) {
+		phys_addr_t phys_addr;
+
+		if (!va || !(va->flags & VM_VM_AREA) || !va->vm)
+			continue;
+
+		if (!(va->vm->flags & VM_IOREMAP))
+			continue;
+
+		phys_addr = va->vm->phys_addr;
+
+		off = (paddr & PAGE_MASK) - (phys_addr & PAGE_MASK);
+		if (off < 0)
+			continue;
+
+		if (off + size <= va->vm->size - PAGE_SIZE) {
+			*offset = off + (paddr & ~PAGE_MASK);
+			vm_area_used_inc(va->vm);
+			rcu_read_unlock();
+			return va->vm;
+		}
+	}
+	rcu_read_unlock();
+
+	return NULL;
+}
+
 /**
  *	find_vm_area  -  find a continuous kernel virtual area
  *	@addr:		base address
@@ -1443,6 +1522,9 @@ static void __vunmap(const void *addr, int deallocate_pages)
 			addr))
 		return;
 
+	if (vm_area_used_dec(addr))
+		return;
+
 	area = remove_vm_area(addr);
 	if (unlikely(!area)) {
 		WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
-- 
1.8.4

[PATCHv2 2/2] ARM: ioremap: Add IO mapping space reused support.

[Richard Lee]

For the IO mapping, the same physical address space maybe
mapped more than one time, for example, in some SoCs:
  - 0x20001000 ~ 0x20001400 --> 1KB for Dev1
  - 0x20001400 ~ 0x20001800 --> 1KB for Dev2
  and the page size is 4KB.

Then both Dev1 and Dev2 will do ioremap operations, and the IO
vmalloc area's virtual address will be aligned down to 4KB, and
the size will be aligned up to 4KB. That's to say, only one
4KB size's vmalloc area could contain Dev1 and Dev2 IO mapping area
at the same time.

For this case, we can ioremap only one time, and the later ioremap
operation will just return the exist vmalloc area.

This patch add IO mapping space reused support.

Signed-off-by: Richard Lee <superlibj8301@gmail.com>
---
 arch/arm/mm/ioremap.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/arm/mm/ioremap.c b/arch/arm/mm/ioremap.c
index f9c32ba..be69333 100644
--- a/arch/arm/mm/ioremap.c
+++ b/arch/arm/mm/ioremap.c
@@ -301,6 +301,12 @@ void __iomem * __arm_ioremap_pfn_caller(unsigned long pfn,
 	if (WARN_ON(pfn_valid(pfn)))
 		return NULL;
 
+	area = find_vm_area_paddr(paddr, size, &offset, VM_IOREMAP);
+	if (area) {
+		addr = (unsigned long)area->addr;
+		return (void __iomem *)(offset + addr);
+	}
+
 	area = get_vm_area_caller(size, VM_IOREMAP, caller);
  	if (!area)
  		return NULL;
-- 
1.8.4

[PATCHv2 1/2] mm/vmalloc: Add IO mapping space reused interface support.

[Rob Herring]

Adding Nico...

On Wed, May 14, 2014 at 3:18 AM, Richard Lee <superlibj8301@gmail.com> wrote:
> For the IO mapping, the same physical address space maybe
> mapped more than one time, for example, in some SoCs:
>   - 0x20001000 ~ 0x20001400 --> 1KB for Dev1
>   - 0x20001400 ~ 0x20001800 --> 1KB for Dev2
>   and the page size is 4KB.
>
> Then both Dev1 and Dev2 will do ioremap operations, and the IO
> vmalloc area's virtual address will be aligned down to 4KB, and
> the size will be aligned up to 4KB. That's to say, only one
> 4KB size's vmalloc area could contain Dev1 and Dev2 IO mapping area
> at the same time.
>
> For this case, we can ioremap only one time, and the later ioremap
> operation will just return the exist vmalloc area.

We already have similar reuse tracking for the static mappings on ARM.
I'm wondering if either that can be reused for ioremap as well or this
can replace the static mapping tracking. It seems silly to have 2
lists to search for finding an existing mapping.

But there is a fundamental problem with your approach. You do not and
cannot check the memory type of the mapping. If you look at the static
mapping code, it only reuses mappings if the memory type match. While
having aliases with different memory types is bad on ARMv6+, I assume
there are some cases that do that given the static mapping code
handles that case. We would at least want to detect and warn if we
didn't want to allow different attributes rather than just return a
mapping with whatever attributes the first mapping used.

Rob

[PATCHv2 1/2] mm/vmalloc: Add IO mapping space reused interface support.

[Andrew Morton]

On Wed, 14 May 2014 16:18:51 +0800 Richard Lee <superlibj8301@gmail.com> wrote:

> For the IO mapping, the same physical address space maybe
> mapped more than one time, for example, in some SoCs:
>   - 0x20001000 ~ 0x20001400 --> 1KB for Dev1
>   - 0x20001400 ~ 0x20001800 --> 1KB for Dev2
>   and the page size is 4KB.
> 
> Then both Dev1 and Dev2 will do ioremap operations, and the IO
> vmalloc area's virtual address will be aligned down to 4KB, and
> the size will be aligned up to 4KB. That's to say, only one
> 4KB size's vmalloc area could contain Dev1 and Dev2 IO mapping area
> at the same time.

Unclear.  What happens when a caller does the two ioremaps at present? 
It fails?  Returns the current mapping's address?  Something else?

> For this case, we can ioremap only one time, and the later ioremap
> operation will just return the exist vmalloc area.

I guess an alternative is to establish a new vmap pointing at the same
physical address.  How does this approach compare to refcounting the
existing vmap?

> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -1,6 +1,7 @@
>  #ifndef _LINUX_VMALLOC_H
>  #define _LINUX_VMALLOC_H
>  
> +#include <linux/atomic.h>
>  #include <linux/spinlock.h>
>  #include <linux/init.h>
>  #include <linux/list.h>
> @@ -34,6 +35,7 @@ struct vm_struct {
>  	struct page		**pages;
>  	unsigned int		nr_pages;
>  	phys_addr_t		phys_addr;
> +	atomic_t		used;
>  	const void		*caller;
>  };
>  
> @@ -100,6 +102,9 @@ static inline size_t get_vm_area_size(const struct vm_struct *area)
>  	return area->size - PAGE_SIZE;
>  }
>  
> +extern struct vm_struct *find_vm_area_paddr(phys_addr_t paddr, size_t size,
> +				     unsigned long *offset,
> +				     unsigned long flags);
>  extern struct vm_struct *get_vm_area(unsigned long size, unsigned long flags);
>  extern struct vm_struct *get_vm_area_caller(unsigned long size,
>  					unsigned long flags, const void *caller);
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index bf233b2..cf0093c 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1293,6 +1293,7 @@ static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
>  	vm->addr = (void *)va->va_start;
>  	vm->size = va->va_end - va->va_start;
>  	vm->caller = caller;
> +	atomic_set(&vm->used, 1);
>  	va->vm = vm;
>  	va->flags |= VM_VM_AREA;
>  	spin_unlock(&vmap_area_lock);
> @@ -1383,6 +1384,84 @@ struct vm_struct *get_vm_area_caller(unsigned long size, unsigned long flags,
>  				  NUMA_NO_NODE, GFP_KERNEL, caller);
>  }
>  
> +static int vm_area_used_inc(struct vm_struct *area)
> +{
> +	if (!(area->flags & VM_IOREMAP))
> +		return -EINVAL;

afaict this can never happen?

> +	atomic_add(1, &area->used);
> +
> +	return atomic_read(&va->vm->used);

atomic_add_return() is neater.  But the return value is in fact never
used so it could return void.

> +}
> +
> +static int vm_area_used_dec(const void *addr)
> +{
> +	struct vmap_area *va;
> +
> +	va = find_vmap_area((unsigned long)addr);
> +	if (!va || !(va->flags & VM_VM_AREA))
> +		return 0;
> +
> +	if (!(va->vm->flags & VM_IOREMAP))
> +		return 0;
> +
> +	atomic_sub(1, &va->vm->used);
> +
> +	return atomic_read(&va->vm->used);

atomic_sub_return()

> +}
> +
> +/**
> + *	find_vm_area_paddr  -  find a continuous kernel virtual area using the
> + *			physical addreess.
> + *	@paddr:		base physical address
> + *	@size:		size of the physical area range
> + *	@offset:	the start offset of the vm area
> + *	@flags:		%VM_IOREMAP for I/O mappings
> + *
> + *	Search for the kernel VM area, whoes physical address starting at
> + *	@paddr, and if the exsit VM area's size is large enough, then return
> + *	it with increasing the 'used' counter, or return NULL.
> + */
> +struct vm_struct *find_vm_area_paddr(phys_addr_t paddr, size_t size,
> +				     unsigned long *offset,
> +				     unsigned long flags)
> +{
> +	struct vmap_area *va;
> +	int off;
> +
> +	if (!(flags & VM_IOREMAP))
> +		return NULL;
> +
> +	size = PAGE_ALIGN((paddr & ~PAGE_MASK) + size);
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(va, &vmap_area_list, list) {
> +		phys_addr_t phys_addr;
> +
> +		if (!va || !(va->flags & VM_VM_AREA) || !va->vm)
> +			continue;
> +
> +		if (!(va->vm->flags & VM_IOREMAP))
> +			continue;
> +
> +		phys_addr = va->vm->phys_addr;
> +
> +		off = (paddr & PAGE_MASK) - (phys_addr & PAGE_MASK);
> +		if (off < 0)
> +			continue;
> +
> +		if (off + size <= va->vm->size - PAGE_SIZE) {
> +			*offset = off + (paddr & ~PAGE_MASK);
> +			vm_area_used_inc(va->vm);
> +			rcu_read_unlock();
> +			return va->vm;
> +		}
> +	}
> +	rcu_read_unlock();
> +
> +	return NULL;
> +}
> +
>  /**
>   *	find_vm_area  -  find a continuous kernel virtual area
>   *	@addr:		base address
> @@ -1443,6 +1522,9 @@ static void __vunmap(const void *addr, int deallocate_pages)
>  			addr))
>  		return;
>  
> +	if (vm_area_used_dec(addr))
> +		return;

This could do with a comment explaining why we return - ie, document
the overall concept/design.

Also, what prevents races here?  Some other thread comes in and grabs a
new reference just after this thread has decided to nuke the vmap?

If there's locking here which I failed to notice then some code
commentary which explains the locking rules would also be nice.

>  	area = remove_vm_area(addr);
>  	if (unlikely(!area)) {
>  		WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",

[PATCHv2 1/2] mm/vmalloc: Add IO mapping space reused interface support.

[Richard Lee]

On Thu, May 15, 2014 at 12:06 AM, Rob Herring <robherring2@gmail.com> wrote:
> Adding Nico...
>
> On Wed, May 14, 2014 at 3:18 AM, Richard Lee <superlibj8301@gmail.com> wrote:
>> For the IO mapping, the same physical address space maybe
>> mapped more than one time, for example, in some SoCs:
>>   - 0x20001000 ~ 0x20001400 --> 1KB for Dev1
>>   - 0x20001400 ~ 0x20001800 --> 1KB for Dev2
>>   and the page size is 4KB.
>>
>> Then both Dev1 and Dev2 will do ioremap operations, and the IO
>> vmalloc area's virtual address will be aligned down to 4KB, and
>> the size will be aligned up to 4KB. That's to say, only one
>> 4KB size's vmalloc area could contain Dev1 and Dev2 IO mapping area
>> at the same time.
>>
>> For this case, we can ioremap only one time, and the later ioremap
>> operation will just return the exist vmalloc area.
>
> We already have similar reuse tracking for the static mappings on ARM.
> I'm wondering if either that can be reused for ioremap as well or this
> can replace the static mapping tracking. It seems silly to have 2
> lists to search for finding an existing mapping.
>

Yes, it is.

Maybe there is one way to reuse the ARM's static mapping tracking, or one
new global way to replace it.



> But there is a fundamental problem with your approach. You do not and
> cannot check the memory type of the mapping. If you look at the static
> mapping code, it only reuses mappings if the memory type match. While
> having aliases with different memory types is bad on ARMv6+, I assume
> there are some cases that do that given the static mapping code
> handles that case. We would at least want to detect and warn if we
> didn't want to allow different attributes rather than just return a
> mapping with whatever attributes the first mapping used.
>

You are right, maybe an alternative is defining one call back for each
individual
ARCH or other ways.


Thanks very much for you comments, they are very important for me.

BRs,
Richard Lee


> Rob

[PATCHv2 1/2] mm/vmalloc: Add IO mapping space reused interface support.

[Richard Lee]

On Thu, May 15, 2014 at 6:56 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Wed, 14 May 2014 16:18:51 +0800 Richard Lee <superlibj8301@gmail.com> wrote:
>
>> For the IO mapping, the same physical address space maybe
>> mapped more than one time, for example, in some SoCs:
>>   - 0x20001000 ~ 0x20001400 --> 1KB for Dev1
>>   - 0x20001400 ~ 0x20001800 --> 1KB for Dev2
>>   and the page size is 4KB.
>>
>> Then both Dev1 and Dev2 will do ioremap operations, and the IO
>> vmalloc area's virtual address will be aligned down to 4KB, and
>> the size will be aligned up to 4KB. That's to say, only one
>> 4KB size's vmalloc area could contain Dev1 and Dev2 IO mapping area
>> at the same time.
>
> Unclear.  What happens when a caller does the two ioremaps at present?
> It fails?  Returns the current mapping's address?  Something else?
>

For this case, should the later one wait ?
Maybe this patch hasn't consider about this.


>> For this case, we can ioremap only one time, and the later ioremap
>> operation will just return the exist vmalloc area.
>
> I guess an alternative is to establish a new vmap pointing at the same
> physical address.  How does this approach compare to refcounting the
> existing vmap?
>

Yes, I'm also thinking to estabish one new vmap.


>> --- a/include/linux/vmalloc.h
>> +++ b/include/linux/vmalloc.h
>> @@ -1,6 +1,7 @@
>>  #ifndef _LINUX_VMALLOC_H
>>  #define _LINUX_VMALLOC_H
>>
>> +#include <linux/atomic.h>
>>  #include <linux/spinlock.h>
>>  #include <linux/init.h>
>>  #include <linux/list.h>
>> @@ -34,6 +35,7 @@ struct vm_struct {
>>       struct page             **pages;
>>       unsigned int            nr_pages;
>>       phys_addr_t             phys_addr;
>> +     atomic_t                used;
>>       const void              *caller;
>>  };
>>
>> @@ -100,6 +102,9 @@ static inline size_t get_vm_area_size(const struct vm_struct *area)
>>       return area->size - PAGE_SIZE;
>>  }
>>
>> +extern struct vm_struct *find_vm_area_paddr(phys_addr_t paddr, size_t size,
>> +                                  unsigned long *offset,
>> +                                  unsigned long flags);
>>  extern struct vm_struct *get_vm_area(unsigned long size, unsigned long flags);
>>  extern struct vm_struct *get_vm_area_caller(unsigned long size,
>>                                       unsigned long flags, const void *caller);
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index bf233b2..cf0093c 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -1293,6 +1293,7 @@ static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
>>       vm->addr = (void *)va->va_start;
>>       vm->size = va->va_end - va->va_start;
>>       vm->caller = caller;
>> +     atomic_set(&vm->used, 1);
>>       va->vm = vm;
>>       va->flags |= VM_VM_AREA;
>>       spin_unlock(&vmap_area_lock);
>> @@ -1383,6 +1384,84 @@ struct vm_struct *get_vm_area_caller(unsigned long size, unsigned long flags,
>>                                 NUMA_NO_NODE, GFP_KERNEL, caller);
>>  }
>>
>> +static int vm_area_used_inc(struct vm_struct *area)
>> +{
>> +     if (!(area->flags & VM_IOREMAP))
>> +             return -EINVAL;
>
> afaict this can never happen?
>

Yes, it is for now.

>> +     atomic_add(1, &area->used);
>> +
>> +     return atomic_read(&va->vm->used);
>
> atomic_add_return() is neater.  But the return value is in fact never
> used so it could return void.
>

yes, that' fine.

>> +}
>> +
>> +static int vm_area_used_dec(const void *addr)
>> +{
>> +     struct vmap_area *va;
>> +
>> +     va = find_vmap_area((unsigned long)addr);
>> +     if (!va || !(va->flags & VM_VM_AREA))
>> +             return 0;
>> +
>> +     if (!(va->vm->flags & VM_IOREMAP))
>> +             return 0;
>> +
>> +     atomic_sub(1, &va->vm->used);
>> +
>> +     return atomic_read(&va->vm->used);
>
> atomic_sub_return()
>

yes,

>> +}
>> +
>> +/**
>> + *   find_vm_area_paddr  -  find a continuous kernel virtual area using the
>> + *                   physical addreess.
>> + *   @paddr:         base physical address
>> + *   @size:          size of the physical area range
>> + *   @offset:        the start offset of the vm area
>> + *   @flags:         %VM_IOREMAP for I/O mappings
>> + *
>> + *   Search for the kernel VM area, whoes physical address starting at
>> + *   @paddr, and if the exsit VM area's size is large enough, then return
>> + *   it with increasing the 'used' counter, or return NULL.
>> + */
>> +struct vm_struct *find_vm_area_paddr(phys_addr_t paddr, size_t size,
>> +                                  unsigned long *offset,
>> +                                  unsigned long flags)
>> +{
>> +     struct vmap_area *va;
>> +     int off;
>> +
>> +     if (!(flags & VM_IOREMAP))
>> +             return NULL;
>> +
>> +     size = PAGE_ALIGN((paddr & ~PAGE_MASK) + size);
>> +
>> +     rcu_read_lock();
>> +     list_for_each_entry_rcu(va, &vmap_area_list, list) {
>> +             phys_addr_t phys_addr;
>> +
>> +             if (!va || !(va->flags & VM_VM_AREA) || !va->vm)
>> +                     continue;
>> +
>> +             if (!(va->vm->flags & VM_IOREMAP))
>> +                     continue;
>> +
>> +             phys_addr = va->vm->phys_addr;
>> +
>> +             off = (paddr & PAGE_MASK) - (phys_addr & PAGE_MASK);
>> +             if (off < 0)
>> +                     continue;
>> +
>> +             if (off + size <= va->vm->size - PAGE_SIZE) {
>> +                     *offset = off + (paddr & ~PAGE_MASK);
>> +                     vm_area_used_inc(va->vm);
>> +                     rcu_read_unlock();
>> +                     return va->vm;
>> +             }
>> +     }
>> +     rcu_read_unlock();
>> +
>> +     return NULL;
>> +}
>> +
>>  /**
>>   *   find_vm_area  -  find a continuous kernel virtual area
>>   *   @addr:          base address
>> @@ -1443,6 +1522,9 @@ static void __vunmap(const void *addr, int deallocate_pages)
>>                       addr))
>>               return;
>>
>> +     if (vm_area_used_dec(addr))
>> +             return;
>
> This could do with a comment explaining why we return - ie, document
> the overall concept/design.
>
> Also, what prevents races here?  Some other thread comes in and grabs a
> new reference just after this thread has decided to nuke the vmap?
>
> If there's locking here which I failed to notice then some code
> commentary which explains the locking rules would also be nice.
>

I will try to revise this.

Actually, I'm thinking about adding a new rb tree for the ioremap vmalloc
area sorted by physical address ? Then this will be more efficient for
searching.



Thanks very much for you comments.

BRs
Richard Lee



>>       area = remove_vm_area(addr);
>>       if (unlikely(!area)) {
>>               WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
>