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Message-Id: <20140514155610.5a8c64fbff857a22cb8c6e91@linux-foundation.org>
Date: Wed, 14 May 2014 15:56:10 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Richard Lee <superlibj8301@...il.com>
Cc: <linux-mm@...ck.org>, <linux@....linux.org.uk>,
<linux-arm-kernel@...ts.infradead.org>, <arnd@...db.de>,
<robherring2@...il.com>, <lauraa@...eaurora.org>,
<d.hatayama@...fujitsu.com>, <zhangyanfei@...fujitsu.com>,
<liwanp@...ux.vnet.ibm.com>, <iamjoonsoo.kim@....com>,
<hannes@...xchg.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 1/2] mm/vmalloc: Add IO mapping space reused interface
support.
On Wed, 14 May 2014 16:18:51 +0800 Richard Lee <superlibj8301@...il.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",
--
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