[PATCH,RFC] Add gfp_mask to get_vm_area()

[PATCH,RFC] Add gfp_mask to get_vm_area()

[David Gibson]

Dave, please consider this patch.  It renames get_vm_area() to
__get_vm_area() and adds a gfp_mask parameter which is passed on to
kmalloc().  get_vm_area(size,flags) is then defined as as
__get_vm_area(size,flags,GFP_KERNEL) to avoid messing with existing
callers.

We need this in order to sanely make pci_alloc_consistent() (and other
consistent allocation functions) obey the DMA-mapping.txt rules on PPC
embedded machines (specifically the requirement that it be callable
from interrupt context).

diff -urN /home/dgibson/kernel/linuxppc-2.5/include/linux/vmalloc.h linux-bluefish/include/linux/vmalloc.h
--- /home/dgibson/kernel/linuxppc-2.5/include/linux/vmalloc.h	2002-09-20 14:36:15.000000000 +1000
+++ linux-bluefish/include/linux/vmalloc.h	2002-10-01 14:29:10.000000000 +1000
@@ -32,7 +32,8 @@
 /*
  *	Lowlevel-APIs (not for driver use!)
  */
-extern struct vm_struct *get_vm_area(unsigned long size, unsigned long flags);
+extern struct vm_struct *__get_vm_area(unsigned long size, unsigned long flags, int gfp_mask);
+#define get_vm_area(size, flags) __get_vm_area(size, flags, GFP_KERNEL)
 extern struct vm_struct *remove_vm_area(void *addr);
 extern int map_vm_area(struct vm_struct *area, pgprot_t prot,
 			struct page ***pages);
diff -urN /home/dgibson/kernel/linuxppc-2.5/mm/vmalloc.c linux-bluefish/mm/vmalloc.c
--- /home/dgibson/kernel/linuxppc-2.5/mm/vmalloc.c	2002-09-20 14:36:26.000000000 +1000
+++ linux-bluefish/mm/vmalloc.c	2002-10-01 14:30:02.000000000 +1000
@@ -181,21 +181,22 @@
 
 
 /**
- *	get_vm_area  -  reserve a contingous kernel virtual area
+ *	__get_vm_area  -  reserve a contingous kernel virtual area
  *
  *	@size:		size of the area
  *	@flags:		%VM_IOREMAP for I/O mappings or VM_ALLOC
+ *	@gfp_mask:	gfp flags to pass to kmalloc()
  *
  *	Search an area of @size in the kernel virtual mapping area,
  *	and reserved it for out purposes.  Returns the area descriptor
  *	on success or %NULL on failure.
  */
-struct vm_struct *get_vm_area(unsigned long size, unsigned long flags)
+struct vm_struct *__get_vm_area(unsigned long size, unsigned long flags, int gfp_mask)
 {
 	struct vm_struct **p, *tmp, *area;
 	unsigned long addr = VMALLOC_START;
 
-	area = kmalloc(sizeof(*area), GFP_KERNEL);
+	area = kmalloc(sizeof(*area), gfp_mask);
 	if (unlikely(!area))
 		return NULL;
 

-- 
David Gibson			| For every complex problem there is a
david@gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[David S. Miller]

   From: David Gibson <david@gibson.dropbear.id.au>
   Date: Tue, 1 Oct 2002 14:42:26 +1000

   Dave, please consider this patch.

I'm fine with this change, but it needs to go through Linus
and Marcelo not me :-)

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[Andrew Morton]

David Gibson wrote:
> 
> Dave, please consider this patch.  It renames get_vm_area() to
> __get_vm_area() and adds a gfp_mask parameter which is passed on to
> kmalloc().  get_vm_area(size,flags) is then defined as as
> __get_vm_area(size,flags,GFP_KERNEL) to avoid messing with existing
> callers.
> 
> We need this in order to sanely make pci_alloc_consistent() (and other
> consistent allocation functions) obey the DMA-mapping.txt rules on PPC
> embedded machines (specifically the requirement that it be callable
> from interrupt context).
> 

I can look after that for you.  But I'd prefer that you just add the
extra gfp_flags argument to get_vm_area() and update the 16 callers.

You cannot call get_vm_area() from interrupt context at present;
it does write_lock(&vmlist_lock) unsafely.

It would be a bit sad to make vmlist_lock interrupt-safe for this.  Is
there no alternative?

(And what the hell is arch/alpha/mm/init.c:callback_init() doing rewriting
vmlist?  Somebody shoot that code)

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[David Gibson]

On Mon, Sep 30, 2002 at 10:08:00PM -0700, Andrew Morton wrote:
> 
> David Gibson wrote:
> >
> > Dave, please consider this patch.  It renames get_vm_area() to
> > __get_vm_area() and adds a gfp_mask parameter which is passed on to
> > kmalloc().  get_vm_area(size,flags) is then defined as as
> > __get_vm_area(size,flags,GFP_KERNEL) to avoid messing with existing
> > callers.
> >
> > We need this in order to sanely make pci_alloc_consistent() (and other
> > consistent allocation functions) obey the DMA-mapping.txt rules on PPC
> > embedded machines (specifically the requirement that it be callable
> > from interrupt context).
> >
> 
> I can look after that for you.  But I'd prefer that you just add the
> extra gfp_flags argument to get_vm_area() and update the 16 callers.
> 
> You cannot call get_vm_area() from interrupt context at present;
> it does write_lock(&vmlist_lock) unsafely.

Oh crap, I'm an idiot.  I've even seen prototype patches for this one
that changed the write_lock() to write_lock_irq(). Duh.

> It would be a bit sad to make vmlist_lock interrupt-safe for this.  Is
> there no alternative?

I don't see an easy one: PPC 4xx has non-coherent cache, so we have to
mark consistent memory non-cacheable.  We want to make the normal
lowmem mapping use large page TLB entries, so we can't frob the
attribute bits on the pages in place.  That means we need to create a
new, non-cacheable mapping for the physical RAM we allocate, which in
turn means allocating a chunk of kernel virtual memory.

> (And what the hell is arch/alpha/mm/init.c:callback_init() doing rewriting
> vmlist?  Somebody shoot that code)

-- 
David Gibson			| For every complex problem there is a
david@gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[Russell King]

On Tue, Oct 01, 2002 at 03:34:17PM +1000, David Gibson wrote:
> I don't see an easy one: PPC 4xx has non-coherent cache, so we have to
> mark consistent memory non-cacheable.  We want to make the normal
> lowmem mapping use large page TLB entries, so we can't frob the
> attribute bits on the pages in place.  That means we need to create a
> new, non-cacheable mapping for the physical RAM we allocate, which in
> turn means allocating a chunk of kernel virtual memory.

Same problem on ARM.  I just haven't got the motivation to rewrite the
bits of the kernel that need to be rewritten to make it work.

Have you checked that your pte/pmd allocation functions can be called
from IRQ context as well?

You basically need:

 - irq-safe get_vm_area
 - irq-safe pmd_alloc_kernel
 - irq-safe pte_alloc_kernel

Last time I looked, all the above were not irq-safe.

-- 
Russell King (rmk@arm.linux.org.uk)                The developer of ARM Linux
             http://www.arm.linux.org.uk/personal/aboutme.html

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[David Gibson]

On Tue, Oct 01, 2002 at 09:42:02AM +0100, Russell King wrote:
> 
> On Tue, Oct 01, 2002 at 03:34:17PM +1000, David Gibson wrote:
> > I don't see an easy one: PPC 4xx has non-coherent cache, so we have to
> > mark consistent memory non-cacheable.  We want to make the normal
> > lowmem mapping use large page TLB entries, so we can't frob the
> > attribute bits on the pages in place.  That means we need to create a
> > new, non-cacheable mapping for the physical RAM we allocate, which in
> > turn means allocating a chunk of kernel virtual memory.
> 
> Same problem on ARM.  I just haven't got the motivation to rewrite the
> bits of the kernel that need to be rewritten to make it work.
> 
> Have you checked that your pte/pmd allocation functions can be called
> from IRQ context as well?
> 
> You basically need:
> 
>  - irq-safe get_vm_area
>  - irq-safe pmd_alloc_kernel
>  - irq-safe pte_alloc_kernel
> 
> Last time I looked, all the above were not irq-safe.

No, I realise they're not safe at the moment - I'm working on that
too.

-- 
David Gibson			| For every complex problem there is a
david@gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[David Gibson]

On Tue, Oct 01, 2002 at 03:34:17PM +1000, David Gibson wrote:
> 
> On Mon, Sep 30, 2002 at 10:08:00PM -0700, Andrew Morton wrote:
> >
> > David Gibson wrote:
> > >
> > > Dave, please consider this patch.  It renames get_vm_area() to
> > > __get_vm_area() and adds a gfp_mask parameter which is passed on to
> > > kmalloc().  get_vm_area(size,flags) is then defined as as
> > > __get_vm_area(size,flags,GFP_KERNEL) to avoid messing with existing
> > > callers.
> > >
> > > We need this in order to sanely make pci_alloc_consistent() (and other
> > > consistent allocation functions) obey the DMA-mapping.txt rules on PPC
> > > embedded machines (specifically the requirement that it be callable
> > > from interrupt context).
> > >
> >
> > I can look after that for you.  But I'd prefer that you just add the
> > extra gfp_flags argument to get_vm_area() and update the 16 callers.
> >
> > You cannot call get_vm_area() from interrupt context at present;
> > it does write_lock(&vmlist_lock) unsafely.
> 
> Oh crap, I'm an idiot.  I've even seen prototype patches for this one
> that changed the write_lock() to write_lock_irq(). Duh.
> 
> > It would be a bit sad to make vmlist_lock interrupt-safe for this.  Is
> > there no alternative?
> 
> I don't see an easy one: PPC 4xx has non-coherent cache, so we have to
> mark consistent memory non-cacheable.  We want to make the normal
> lowmem mapping use large page TLB entries, so we can't frob the
> attribute bits on the pages in place.  That means we need to create a
> new, non-cacheable mapping for the physical RAM we allocate, which in
> turn means allocating a chunk of kernel virtual memory.

Well, here is an updated patch which should make the vmlist_lock
interrupt-safe.  It makes __get_vm_area() and remove_vm_area()
callable from interrupt context.  If you can think of some way of
avoiding doing that, I'd love to hear it - I agree this would be
rather sad.

Thoughts?

Incidentally, DaveM, what's the rationale for requiring the consistent
allocation functions to be callable from interrupt context?  It's not
immediately obvious to me.

diff -urN /home/dgibson/kernel/linuxppc-2.5/fs/proc/kcore.c linux-bluefish/fs/proc/kcore.c
--- /home/dgibson/kernel/linuxppc-2.5/fs/proc/kcore.c	2002-10-01 10:17:33.000000000 +1000
+++ linux-bluefish/fs/proc/kcore.c	2002-10-03 14:25:32.000000000 +1000
@@ -318,10 +318,10 @@
 	int num_vma;
 	unsigned long start;
 
-	read_lock(&vmlist_lock);
+	read_lock_irq(&vmlist_lock);
 	proc_root_kcore->size = size = get_kcore_size(&num_vma, &elf_buflen);
 	if (buflen == 0 || *fpos >= size) {
-		read_unlock(&vmlist_lock);
+		read_unlock_irq(&vmlist_lock);
 		return 0;
 	}
 
@@ -338,12 +338,12 @@
 			tsz = buflen;
 		elf_buf = kmalloc(elf_buflen, GFP_ATOMIC);
 		if (!elf_buf) {
-			read_unlock(&vmlist_lock);
+			read_unlock_irq(&vmlist_lock);
 			return -ENOMEM;
 		}
 		memset(elf_buf, 0, elf_buflen);
 		elf_kcore_store_hdr(elf_buf, num_vma, elf_buflen);
-		read_unlock(&vmlist_lock);
+		read_unlock_irq(&vmlist_lock);
 		if (copy_to_user(buffer, elf_buf + *fpos, tsz)) {
 			kfree(elf_buf);
 			return -EFAULT;
@@ -358,7 +358,7 @@
 		if (buflen == 0)
 			return acc;
 	} else
-		read_unlock(&vmlist_lock);
+		read_unlock_irq(&vmlist_lock);
 
 	/* where page 0 not mapped, write zeros into buffer */
 #if defined (__i386__) || defined (__mc68000__) || defined(__x86_64__)
@@ -403,7 +403,7 @@
 				return -ENOMEM;
 			memset(elf_buf, 0, tsz);
 
-			read_lock(&vmlist_lock);
+			read_lock_irq(&vmlist_lock);
 			for (m=vmlist; m && cursize; m=m->next) {
 				unsigned long vmstart;
 				unsigned long vmsize;
@@ -431,7 +431,7 @@
 				memcpy(elf_buf + (vmstart - start),
 					(char *)vmstart, vmsize);
 			}
-			read_unlock(&vmlist_lock);
+			read_unlock_irq(&vmlist_lock);
 			if (copy_to_user(buffer, elf_buf, tsz)) {
 				kfree(elf_buf);
 				return -EFAULT;
diff -urN /home/dgibson/kernel/linuxppc-2.5/include/linux/vmalloc.h linux-bluefish/include/linux/vmalloc.h
--- /home/dgibson/kernel/linuxppc-2.5/include/linux/vmalloc.h	2002-09-20 14:36:15.000000000 +1000
+++ linux-bluefish/include/linux/vmalloc.h	2002-10-01 14:29:10.000000000 +1000
@@ -32,7 +32,8 @@
 /*
  *	Lowlevel-APIs (not for driver use!)
  */
-extern struct vm_struct *get_vm_area(unsigned long size, unsigned long flags);
+extern struct vm_struct *__get_vm_area(unsigned long size, unsigned long flags, int gfp_mask);
+#define get_vm_area(size, flags) __get_vm_area(size, flags, GFP_KERNEL)
 extern struct vm_struct *remove_vm_area(void *addr);
 extern int map_vm_area(struct vm_struct *area, pgprot_t prot,
 			struct page ***pages);
diff -urN /home/dgibson/kernel/linuxppc-2.5/mm/vmalloc.c linux-bluefish/mm/vmalloc.c
--- /home/dgibson/kernel/linuxppc-2.5/mm/vmalloc.c	2002-09-20 14:36:26.000000000 +1000
+++ linux-bluefish/mm/vmalloc.c	2002-10-03 14:27:40.000000000 +1000
@@ -181,21 +181,23 @@
 
 
 /**
- *	get_vm_area  -  reserve a contingous kernel virtual area
+ *	__get_vm_area  -  reserve a contingous kernel virtual area
  *
  *	@size:		size of the area
  *	@flags:		%VM_IOREMAP for I/O mappings or VM_ALLOC
+ *	@gfp_mask:	gfp flags to pass to kmalloc()
  *
  *	Search an area of @size in the kernel virtual mapping area,
  *	and reserved it for out purposes.  Returns the area descriptor
  *	on success or %NULL on failure.
  */
-struct vm_struct *get_vm_area(unsigned long size, unsigned long flags)
+struct vm_struct *__get_vm_area(unsigned long size, unsigned long flags, int gfp_mask)
 {
 	struct vm_struct **p, *tmp, *area;
 	unsigned long addr = VMALLOC_START;
+	unsigned long flags;
 
-	area = kmalloc(sizeof(*area), GFP_KERNEL);
+	area = kmalloc(sizeof(*area), gfp_mask);
 	if (unlikely(!area))
 		return NULL;
 
@@ -204,7 +206,7 @@
 	 */
 	size += PAGE_SIZE;
 
-	write_lock(&vmlist_lock);
+	write_lock_irqsave(&vmlist_lock, flags);
 	for (p = &vmlist; (tmp = *p) ;p = &tmp->next) {
 		if ((size + addr) < addr)
 			goto out;
@@ -225,12 +227,12 @@
 	area->pages = NULL;
 	area->nr_pages = 0;
 	area->phys_addr = 0;
-	write_unlock(&vmlist_lock);
+	write_unlock_irqrestore(&vmlist_lock, flags);
 
 	return area;
 
 out:
-	write_unlock(&vmlist_lock);
+	write_unlock_irqrestore(&vmlist_lock, flags);
 	kfree(area);
 	return NULL;
 }
@@ -247,18 +249,19 @@
 struct vm_struct *remove_vm_area(void *addr)
 {
 	struct vm_struct **p, *tmp;
+	unsigned long flags;
 
-	write_lock(&vmlist_lock);
+	write_lock_irqsave(&vmlist_lock, flags);
 	for (p = &vmlist ; (tmp = *p) ;p = &tmp->next) {
 		 if (tmp->addr == addr)
 			 goto found;
 	}
-	write_unlock(&vmlist_lock);
+	write_unlock_irqrestore(&vmlist_lock, flags);
 	return NULL;
 
 found:
 	*p = tmp->next;
-	write_unlock(&vmlist_lock);
+	write_unlock_irqrestore(&vmlist_lock, flags);
 	return tmp;
 }
 
@@ -452,7 +455,7 @@
 	if ((unsigned long) addr + count < count)
 		count = -(unsigned long) addr;
 
-	read_lock(&vmlist_lock);
+	read_lock_irq(&vmlist_lock);
 	for (tmp = vmlist; tmp; tmp = tmp->next) {
 		vaddr = (char *) tmp->addr;
 		if (addr >= vaddr + tmp->size - PAGE_SIZE)
@@ -476,7 +479,7 @@
 		} while (--n > 0);
 	}
 finished:
-	read_unlock(&vmlist_lock);
+	read_unlock_irq(&vmlist_lock);
 	return buf - buf_start;
 }
 
@@ -490,7 +493,7 @@
 	if ((unsigned long) addr + count < count)
 		count = -(unsigned long) addr;
 
-	read_lock(&vmlist_lock);
+	read_lock_irq(&vmlist_lock);
 	for (tmp = vmlist; tmp; tmp = tmp->next) {
 		vaddr = (char *) tmp->addr;
 		if (addr >= vaddr + tmp->size - PAGE_SIZE)
@@ -513,6 +516,6 @@
 		} while (--n > 0);
 	}
 finished:
-	read_unlock(&vmlist_lock);
+	read_unlock_irq(&vmlist_lock);
 	return buf - buf_start;
 }


-- 
David Gibson			| For every complex problem there is a
david@gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[David Gibson]

On Thu, Oct 03, 2002 at 02:39:48PM +1000, David Gibson wrote:
> 
> On Tue, Oct 01, 2002 at 03:34:17PM +1000, David Gibson wrote:
> >
> > On Mon, Sep 30, 2002 at 10:08:00PM -0700, Andrew Morton wrote:
> > >
> > > David Gibson wrote:
> > > >
> > > > Dave, please consider this patch.  It renames get_vm_area() to
> > > > __get_vm_area() and adds a gfp_mask parameter which is passed on to
> > > > kmalloc().  get_vm_area(size,flags) is then defined as as
> > > > __get_vm_area(size,flags,GFP_KERNEL) to avoid messing with existing
> > > > callers.
> > > >
> > > > We need this in order to sanely make pci_alloc_consistent() (and other
> > > > consistent allocation functions) obey the DMA-mapping.txt rules on PPC
> > > > embedded machines (specifically the requirement that it be callable
> > > > from interrupt context).
> > > >
> > >
> > > I can look after that for you.  But I'd prefer that you just add the
> > > extra gfp_flags argument to get_vm_area() and update the 16 callers.
> > >
> > > You cannot call get_vm_area() from interrupt context at present;
> > > it does write_lock(&vmlist_lock) unsafely.
> >
> > Oh crap, I'm an idiot.  I've even seen prototype patches for this one
> > that changed the write_lock() to write_lock_irq(). Duh.
> >
> > > It would be a bit sad to make vmlist_lock interrupt-safe for this.  Is
> > > there no alternative?
> >
> > I don't see an easy one: PPC 4xx has non-coherent cache, so we have to
> > mark consistent memory non-cacheable.  We want to make the normal
> > lowmem mapping use large page TLB entries, so we can't frob the
> > attribute bits on the pages in place.  That means we need to create a
> > new, non-cacheable mapping for the physical RAM we allocate, which in
> > turn means allocating a chunk of kernel virtual memory.
> 
> Well, here is an updated patch which should make the vmlist_lock
> interrupt-safe.  It makes __get_vm_area() and remove_vm_area()
> callable from interrupt context.  If you can think of some way of
> avoiding doing that, I'd love to hear it - I agree this would be
> rather sad.

Blah.  It gets worse.  Making map_page() or remap_page_range()
interrupt-safe would require making mm->page_table_lock irq-safe too
:-(

Maybe non-coherent architectures should should pre-allocate a chunk of
virtual memory for consistent allocations, and pre-allocate all its
page tables.

-- 
David Gibson			| For every complex problem there is a
david@gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[Russell King]

On Thu, Oct 03, 2002 at 02:56:44PM +1000, David Gibson wrote:
> Blah.  It gets worse.  Making map_page() or remap_page_range()
> interrupt-safe would require making mm->page_table_lock irq-safe too
> :-(
> 
> Maybe non-coherent architectures should should pre-allocate a chunk of
> virtual memory for consistent allocations, and pre-allocate all its
> page tables.

There are a growing number of applications out there for ARM stuff where
this would be impractical.  Those wanting about 3GB of kernel space vs
1GB user space.

Doubling the virtual requirement for the SDRAM will make Linux unusable
in these situations, and then you'll have nice people from Intel and
Montavista banging on your door asking you why you killed their product
line.

The current situation on ARM works for 95% of cases.  If the choice is
between "95% working" and "cutting off the hand that feeds you" I'd
prefer the former.

On a more constructive note, I believe there is a way around the
mm->page_table_lock problem.  I believe we should completely split the
handling of the user space page tables from the kernel space page tables.

User space can carry on using mm->page_table_lock and be happy; it should
never ever touch the kernel page tables.

We then only have to worry about making things that touch the kernel page
tables irq-safe.  How many of those are there?  Two.  ioremap and vmalloc.
Neither of these two functions has any business touching anything other
than pid0's tables, and certainly has no business touching user space
page tables.  The problem is now far easier to deal with.

remap_page_range() shouldn't be a problem - its supposed to map pages
into user space, and if you're calling that from IRQ context, you're
doing something really wrong.

If I can get out of my current circle of never-ending problems and paid-
for work on other areas of ARM stuff, I might be able to look at this.
I've currently got an estimated backlog of one whole week on anything I
do atm.

-- 
Russell King (rmk@arm.linux.org.uk)                The developer of ARM Linux
             http://www.arm.linux.org.uk/personal/aboutme.html

Re: [PATCH,RFC] Add gfp_mask to get_vm_area()

[David Gibson]

On Thu, Oct 03, 2002 at 04:18:16PM +0100, Russell King wrote:
> 
> On Thu, Oct 03, 2002 at 02:56:44PM +1000, David Gibson wrote:
> > Blah.  It gets worse.  Making map_page() or remap_page_range()
> > interrupt-safe would require making mm->page_table_lock irq-safe too
> > :-(
> >
> > Maybe non-coherent architectures should should pre-allocate a chunk of
> > virtual memory for consistent allocations, and pre-allocate all its
> > page tables.
> 
> There are a growing number of applications out there for ARM stuff where
> this would be impractical.  Those wanting about 3GB of kernel space vs
> 1GB user space.
> 
> Doubling the virtual requirement for the SDRAM will make Linux unusable
> in these situations, and then you'll have nice people from Intel and
> Montavista banging on your door asking you why you killed their product
> line.

Well I certainly wasn't we have an additional virtual mapping for all
of physical RAM - if nothing else that would waste a bunch of space in
page tables.  I was thinking of a pool of VM out of which to establish
new consistent mappings - with a simple and irq-safe allocated working
within the pre-allocated chunk of VM.

Which of course raises the hard question: how big should the pool be?
It would be possible to have something running in user context which
tops up the pool if it is low, but that starts getting complex, and
wouldn't help a driver which wants to allocate a large chunk of
consistent memory from interrupt context.

> The current situation on ARM works for 95% of cases.  If the choice is
> between "95% working" and "cutting off the hand that feeds you" I'd
> prefer the former.
> 
> On a more constructive note, I believe there is a way around the
> mm->page_table_lock problem.  I believe we should completely split the
> handling of the user space page tables from the kernel space page tables.
> 
> User space can carry on using mm->page_table_lock and be happy; it should
> never ever touch the kernel page tables.
> 
> We then only have to worry about making things that touch the kernel page
> tables irq-safe.  How many of those are there?  Two.  ioremap and vmalloc.
> Neither of these two functions has any business touching anything other
> than pid0's tables, and certainly has no business touching user space
> page tables.  The problem is now far easier to deal with.

Well, that sounds like it would work.  On the other hand, it seems
like a non-trivial set of modifications to the VM structure.

> remap_page_range() shouldn't be a problem - its supposed to map pages
> into user space, and if you're calling that from IRQ context, you're
> doing something really wrong.
> 
> If I can get out of my current circle of never-ending problems and paid-
> for work on other areas of ARM stuff, I might be able to look at this.
> I've currently got an estimated backlog of one whole week on anything I
> do atm.

-- 
David Gibson			| For every complex problem there is a
david@gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson