[PATCH 0/14] Pass MAP_FIXED down to get_unmapped_area

[PATCH 0/14] Pass MAP_FIXED down to get_unmapped_area

[Benjamin Herrenschmidt]

This is a "first step" as there are still cleanups to be done in various
areas touched by that code but I think it's probably good to go as is and
at least enables me to implement what I need for PowerPC.

The current get_unmapped_area code calls the f_ops->get_unmapped_area or
the arch one (via the mm) only when MAP_FIXED is not passed. That makes
it impossible for archs to impose proper constraints on regions of the
virtual address space. To work around that, get_unmapped_area() then
calls some hugetlbfs specific hacks.

This cause several problems, among others:

 - It makes it impossible for a driver or filesystem to do the same thing
that hugetlbfs does (for example, to allow a driver to use larger page
sizes to map external hardware) if that requires applying a constraint
on the addresses (constraining that mapping in certain regions and other
mappings out of those regions).

 - Some archs like arm, mips, sparc, sparc64, sh and sh64 already want
MAP_FIXED to be passed down in order to deal with aliasing issues.
The code is there to handle it... but is never called.

This serie of patches moves the logic to handle MAP_FIXED down to the
various arch/driver get_unmapped_area() implementations, and then changes
the generic code to always call them. The hugetlbfs hacks then disappear
from the generic code.

Since I need to do some special 64K pages mappings for SPEs on cell, I need
to work around the first problem at least. I have further patches thus
implementing a "slices" layer that handles multiple page sizes through
slices of the address space for use by hugetlbfs, the SPE code, and possibly
others, but it requires that serie of patches first/

There is still a potential (but not practical) issue due to the fact that
filesystems/drivers implemeting g_u_a will effectively bypass all arch
checks. This is not an issue in practice as the only filesystems/drivers
using that hook are doing so for arch specific purposes in the first place.

There is also a problem with mremap that will completely bypass all arch
checks. I'll try to address that separately, I'm not 100% certain yet how,
possibly by making it not work when the vma has a file whose f_ops has a
get_unmapped_area callback, and by making it use is_hugepage_only_range()
before expanding into a new area.

Also, I want to turn is_hugepage_only_range() into a more generic
is_normal_page_range() as that's really what it will end up meaning
when used in stack grow, brk grow and mremap.

None of the above "issues" however are introduced by this patch, they are
already there, so I think the patch can go in after review of the individual
bits by respective maintainers I suppose.

Cheers,
Ben.

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[PATCH 1/14] get_unmapped_area handles MAP_FIXED on powerpc

[Benjamin Herrenschmidt]

---

 arch/powerpc/mm/hugetlbpage.c |   21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

Index: linux-cell/arch/powerpc/mm/hugetlbpage.c
===================================================================
--- linux-cell.orig/arch/powerpc/mm/hugetlbpage.c	2007-03-22 14:52:07.000000000 +1100
+++ linux-cell/arch/powerpc/mm/hugetlbpage.c	2007-03-22 14:57:40.000000000 +1100
@@ -572,6 +572,13 @@ unsigned long arch_get_unmapped_area(str
 	if (len > TASK_SIZE)
 		return -ENOMEM;
 
+	/* handle fixed mapping: prevent overlap with huge pages */
+	if (flags & MAP_FIXED) {
+		if (is_hugepage_only_range(mm, addr, len))
+			return -EINVAL;
+		return addr;
+	}
+
 	if (addr) {
 		addr = PAGE_ALIGN(addr);
 		vma = find_vma(mm, addr);
@@ -647,6 +654,13 @@ arch_get_unmapped_area_topdown(struct fi
 	if (len > TASK_SIZE)
 		return -ENOMEM;
 
+	/* handle fixed mapping: prevent overlap with huge pages */
+	if (flags & MAP_FIXED) {
+		if (is_hugepage_only_range(mm, addr, len))
+			return -EINVAL;
+		return addr;
+	}
+
 	/* dont allow allocations above current base */
 	if (mm->free_area_cache > base)
 		mm->free_area_cache = base;
@@ -829,6 +843,13 @@ unsigned long hugetlb_get_unmapped_area(
 	/* Paranoia, caller should have dealt with this */
 	BUG_ON((addr + len)  < addr);
 
+	/* Handle MAP_FIXED */
+	if (flags & MAP_FIXED) {
+		if (prepare_hugepage_range(addr, len, pgoff))
+			return -EINVAL;
+		return addr;
+	}
+
 	if (test_thread_flag(TIF_32BIT)) {
 		curareas = current->mm->context.low_htlb_areas;
 

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[PATCH 2/14] get_unmapped_area handles MAP_FIXED on alpha

[Benjamin Herrenschmidt]

---

 arch/alpha/kernel/osf_sys.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux-cell/arch/alpha/kernel/osf_sys.c
===================================================================
--- linux-cell.orig/arch/alpha/kernel/osf_sys.c	2007-03-22 14:58:33.000000000 +1100
+++ linux-cell/arch/alpha/kernel/osf_sys.c	2007-03-22 14:58:44.000000000 +1100
@@ -1267,6 +1267,9 @@ arch_get_unmapped_area(struct file *filp
 	if (len > limit)
 		return -ENOMEM;
 
+	if (flags & MAP_FIXED)
+		return addr;
+
 	/* First, see if the given suggestion fits.
 
 	   The OSF/1 loader (/sbin/loader) relies on us returning an

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[PATCH 3/14] get_unmapped_area handles MAP_FIXED on arm

[Benjamin Herrenschmidt]

---

 arch/arm/mm/mmap.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

Index: linux-cell/arch/arm/mm/mmap.c
===================================================================
--- linux-cell.orig/arch/arm/mm/mmap.c	2007-03-22 14:59:51.000000000 +1100
+++ linux-cell/arch/arm/mm/mmap.c	2007-03-22 15:00:01.000000000 +1100
@@ -49,8 +49,7 @@ arch_get_unmapped_area(struct file *filp
 #endif
 
 	/*
-	 * We should enforce the MAP_FIXED case.  However, currently
-	 * the generic kernel code doesn't allow us to handle this.
+	 * We enforce the MAP_FIXED case.
 	 */
 	if (flags & MAP_FIXED) {
 		if (aliasing && flags & MAP_SHARED && addr & (SHMLBA - 1))

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[PATCH 4/14] get_unmapped_area handles MAP_FIXED on frv

[Benjamin Herrenschmidt]

---

 arch/frv/mm/elf-fdpic.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux-cell/arch/frv/mm/elf-fdpic.c
===================================================================
--- linux-cell.orig/arch/frv/mm/elf-fdpic.c	2007-03-22 15:00:50.000000000 +1100
+++ linux-cell/arch/frv/mm/elf-fdpic.c	2007-03-22 15:01:06.000000000 +1100
@@ -64,6 +64,10 @@ unsigned long arch_get_unmapped_area(str
 	if (len > TASK_SIZE)
 		return -ENOMEM;
 
+	/* handle MAP_FIXED */
+	if (flags & MAP_FIXED)
+		return addr;
+
 	/* only honour a hint if we're not going to clobber something doing so */
 	if (addr) {
 		addr = PAGE_ALIGN(addr);

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[PATCH 6/14] get_unmapped_area handles MAP_FIXED on ia64

[Benjamin Herrenschmidt]

---

 arch/ia64/kernel/sys_ia64.c |    7 +++++++
 arch/ia64/mm/hugetlbpage.c  |    8 ++++++++
 2 files changed, 15 insertions(+)

Index: linux-cell/arch/ia64/kernel/sys_ia64.c
===================================================================
--- linux-cell.orig/arch/ia64/kernel/sys_ia64.c	2007-03-22 15:10:45.000000000 +1100
+++ linux-cell/arch/ia64/kernel/sys_ia64.c	2007-03-22 15:10:47.000000000 +1100
@@ -33,6 +33,13 @@ arch_get_unmapped_area (struct file *fil
 	if (len > RGN_MAP_LIMIT)
 		return -ENOMEM;
 
+	/* handle fixed mapping: prevent overlap with huge pages */
+	if (flags & MAP_FIXED) {
+		if (is_hugepage_only_range(mm, addr, len))
+			return -EINVAL;
+		return addr;
+	}
+
 #ifdef CONFIG_HUGETLB_PAGE
 	if (REGION_NUMBER(addr) == RGN_HPAGE)
 		addr = 0;
Index: linux-cell/arch/ia64/mm/hugetlbpage.c
===================================================================
--- linux-cell.orig/arch/ia64/mm/hugetlbpage.c	2007-03-22 15:12:32.000000000 +1100
+++ linux-cell/arch/ia64/mm/hugetlbpage.c	2007-03-22 15:12:39.000000000 +1100
@@ -148,6 +148,14 @@ unsigned long hugetlb_get_unmapped_area(
 		return -ENOMEM;
 	if (len & ~HPAGE_MASK)
 		return -EINVAL;
+
+	/* Handle MAP_FIXED */
+	if (flags & MAP_FIXED) {
+		if (prepare_hugepage_range(addr, len, pgoff))
+			return -EINVAL;
+		return addr;
+	}
+
 	/* This code assumes that RGN_HPAGE != 0. */
 	if ((REGION_NUMBER(addr) != RGN_HPAGE) || (addr & (HPAGE_SIZE - 1)))
 		addr = HPAGE_REGION_BASE;
 

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[PATCH 5/14] get_unmapped_area handles MAP_FIXED on i386

[Benjamin Herrenschmidt]

---

 arch/i386/mm/hugetlbpage.c |    6 ++++++
 1 file changed, 6 insertions(+)

Index: linux-cell/arch/i386/mm/hugetlbpage.c
===================================================================
--- linux-cell.orig/arch/i386/mm/hugetlbpage.c	2007-03-22 16:08:12.000000000 +1100
+++ linux-cell/arch/i386/mm/hugetlbpage.c	2007-03-22 16:14:19.000000000 +1100
@@ -367,6 +367,12 @@ hugetlb_get_unmapped_area(struct file *f
 	if (len > TASK_SIZE)
 		return -ENOMEM;
 
+	if (flags & MAP_FIXED) {
+		if (prepare_hugepage_range(addr, len, pgoff))
+			return -EINVAL;
+		return addr;
+	}
+
 	if (addr) {
 		addr = ALIGN(addr, HPAGE_SIZE);
 		vma = find_vma(mm, addr);

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[PATCH 7/14] get_unmapped_area handles MAP_FIXED on parisc

[Benjamin Herrenschmidt]

---

 arch/parisc/kernel/sys_parisc.c |    5 +++++
 1 file changed, 5 insertions(+)

Index: linux-cell/arch/parisc/kernel/sys_parisc.c
===================================================================
--- linux-cell.orig/arch/parisc/kernel/sys_parisc.c	2007-03-22 15:28:05.000000000 +1100
+++ linux-cell/arch/parisc/kernel/sys_parisc.c	2007-03-22 15:29:08.000000000 +1100
@@ -106,6 +106,11 @@ unsigned long arch_get_unmapped_area(str
 {
 	if (len > TASK_SIZE)
 		return -ENOMEM;
+	/* Might want to check for cache aliasing issues for MAP_FIXED case
+	 * like ARM or MIPS ??? --BenH.
+	 */
+	if (flags & MAP_FIXED)
+		return addr;
 	if (!addr)
 		addr = TASK_UNMAPPED_BASE;
 

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[PATCH 8/14] get_unmapped_area handles MAP_FIXED on sparc64

[Benjamin Herrenschmidt]

---

 arch/sparc64/mm/hugetlbpage.c |    6 ++++++
 1 file changed, 6 insertions(+)

Index: linux-cell/arch/sparc64/mm/hugetlbpage.c
===================================================================
--- linux-cell.orig/arch/sparc64/mm/hugetlbpage.c	2007-03-22 16:12:57.000000000 +1100
+++ linux-cell/arch/sparc64/mm/hugetlbpage.c	2007-03-22 16:15:33.000000000 +1100
@@ -175,6 +175,12 @@ hugetlb_get_unmapped_area(struct file *f
 	if (len > task_size)
 		return -ENOMEM;
 
+	if (flags & MAP_FIXED) {
+		if (prepare_hugepage_range(addr, len, pgoff))
+			return -EINVAL;
+		return addr;
+	}
+
 	if (addr) {
 		addr = ALIGN(addr, HPAGE_SIZE);
 		vma = find_vma(mm, addr);

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[PATCH 9/14] get_unmapped_area handles MAP_FIXED on x86_64

[Benjamin Herrenschmidt]

---

 arch/x86_64/kernel/sys_x86_64.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux-cell/arch/x86_64/kernel/sys_x86_64.c
===================================================================
--- linux-cell.orig/arch/x86_64/kernel/sys_x86_64.c	2007-03-22 16:10:10.000000000 +1100
+++ linux-cell/arch/x86_64/kernel/sys_x86_64.c	2007-03-22 16:11:06.000000000 +1100
@@ -93,6 +93,9 @@ arch_get_unmapped_area(struct file *filp
 	unsigned long start_addr;
 	unsigned long begin, end;
 	
+	if (flags & MAP_FIXED)
+		return addr;
+
 	find_start_end(flags, &begin, &end); 
 
 	if (len > end)

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[PATCH 10/14] get_unmapped_area handles MAP_FIXED in hugetlbfs

[Benjamin Herrenschmidt]

---

 fs/hugetlbfs/inode.c |    6 ++++++
 1 file changed, 6 insertions(+)

Index: linux-cell/fs/hugetlbfs/inode.c
===================================================================
--- linux-cell.orig/fs/hugetlbfs/inode.c	2007-03-22 16:12:56.000000000 +1100
+++ linux-cell/fs/hugetlbfs/inode.c	2007-03-22 16:16:02.000000000 +1100
@@ -115,6 +115,12 @@ hugetlb_get_unmapped_area(struct file *f
 	if (len > TASK_SIZE)
 		return -ENOMEM;
 
+	if (flags & MAP_FIXED) {
+		if (prepare_hugepage_range(addr, len, pgoff))
+			return -EINVAL;
+		return addr;
+	}
+
 	if (addr) {
 		addr = ALIGN(addr, HPAGE_SIZE);
 		vma = find_vma(mm, addr);

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[PATCH 11/14] get_unmapped_area handles MAP_FIXED on ramfs (nommu)

[Benjamin Herrenschmidt]

---

 fs/ramfs/file-nommu.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Index: linux-cell/fs/ramfs/file-nommu.c
===================================================================
--- linux-cell.orig/fs/ramfs/file-nommu.c	2007-03-22 16:18:27.000000000 +1100
+++ linux-cell/fs/ramfs/file-nommu.c	2007-03-22 16:20:14.000000000 +1100
@@ -238,7 +238,10 @@ unsigned long ramfs_nommu_get_unmapped_a
 	struct page **pages = NULL, **ptr, *page;
 	loff_t isize;
 
-	if (!(flags & MAP_SHARED))
+	/* Deal with MAP_FIXED differently ? Forbid it ? Need help from some nommu
+	 * folks there... --BenH.
+	 */
+	if ((flags & MAP_FIXED) || !(flags & MAP_SHARED))
 		return addr;
 
 	/* the mapping mustn't extend beyond the EOF */

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Re: [PATCH 11/14] get_unmapped_area handles MAP_FIXED on ramfs (nommu)

[David Howells]

Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> -	if (!(flags & MAP_SHARED))
> +	/* Deal with MAP_FIXED differently ? Forbid it ? Need help from some nommu
> +	 * folks there... --BenH.
> +	 */
> +	if ((flags & MAP_FIXED) || !(flags & MAP_SHARED))

MAP_FIXED on NOMMU?  Surely you jest...

See the first if-statement in validate_mmap_request().

If anything, you should be adding BUG_ON(flags & MAP_FIXED).

David

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Re: [PATCH 11/14] get_unmapped_area handles MAP_FIXED on ramfs (nommu)

[Benjamin Herrenschmidt]

On Wed, 2007-04-04 at 11:16 +0100, David Howells wrote:
> Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> 
> > -	if (!(flags & MAP_SHARED))
> > +	/* Deal with MAP_FIXED differently ? Forbid it ? Need help from some nommu
> > +	 * folks there... --BenH.
> > +	 */
> > +	if ((flags & MAP_FIXED) || !(flags & MAP_SHARED))
> 
> MAP_FIXED on NOMMU?  Surely you jest...

Heh, see the comment, I was actually wondering about it :-)

> See the first if-statement in validate_mmap_request().
> 
> If anything, you should be adding BUG_ON(flags & MAP_FIXED).

Yeah, I missed that bit. That will simplify the problem.

Thanks,
Ben.


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[PATCH 12/14] get_unmapped_area handles MAP_FIXED in /dev/mem (nommu)

[Benjamin Herrenschmidt]

This also fixes a bug, I think, it used to return a pgoff (pfn)
instead of an address. (To split ?)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

 drivers/char/mem.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Index: linux-cell/drivers/char/mem.c
===================================================================
--- linux-cell.orig/drivers/char/mem.c	2007-03-22 16:24:04.000000000 +1100
+++ linux-cell/drivers/char/mem.c	2007-03-22 16:26:30.000000000 +1100
@@ -246,9 +246,12 @@ static unsigned long get_unmapped_area_m
 					   unsigned long pgoff,
 					   unsigned long flags)
 {
+	if (flags & MAP_FIXED)
+		if ((addr >> PAGE_SHIFT) != pgoff)
+			return (unsigned long) -EINVAL;
 	if (!valid_mmap_phys_addr_range(pgoff, len))
 		return (unsigned long) -EINVAL;
-	return pgoff;
+	return pgoff << PAGE_SHIFT;
 }
 
 /* can't do an in-place private mapping if there's no MMU */

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Re: [PATCH 12/14] get_unmapped_area handles MAP_FIXED in /dev/mem (nommu)

[David Howells]

Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> +	if (flags & MAP_FIXED)
> +		if ((addr >> PAGE_SHIFT) != pgoff)
> +			return (unsigned long) -EINVAL;

Again... in NOMMU-mode there is no MAP_FIXED - it's rejected before we get
this far.

> -	return pgoff;
> +	return pgoff << PAGE_SHIFT;

That, however, does appear to be a genuine bugfix.

David

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Re: [PATCH 12/14] get_unmapped_area handles MAP_FIXED in /dev/mem (nommu)

[Benjamin Herrenschmidt]

On Wed, 2007-04-04 at 11:31 +0100, David Howells wrote:
> Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> 
> > +	if (flags & MAP_FIXED)
> > +		if ((addr >> PAGE_SHIFT) != pgoff)
> > +			return (unsigned long) -EINVAL;
> 
> Again... in NOMMU-mode there is no MAP_FIXED - it's rejected before we get
> this far.
> 
> > -	return pgoff;
> > +	return pgoff << PAGE_SHIFT;
> 
> That, however, does appear to be a genuine bugfix.

I'll separate it from the rest of the patches

Ben.


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[PATCH 13/14] get_unmapped_area handles MAP_FIXED in generic code

[Benjamin Herrenschmidt]

and change the caller now that everybody can handle it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

 mm/mmap.c |   25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

Index: linux-cell/mm/mmap.c
===================================================================
--- linux-cell.orig/mm/mmap.c	2007-03-22 16:29:22.000000000 +1100
+++ linux-cell/mm/mmap.c	2007-03-22 16:30:06.000000000 +1100
@@ -1199,6 +1199,9 @@ arch_get_unmapped_area(struct file *filp
 	if (len > TASK_SIZE)
 		return -ENOMEM;
 
+	if (flags & MAP_FIXED)
+		return addr;
+
 	if (addr) {
 		addr = PAGE_ALIGN(addr);
 		vma = find_vma(mm, addr);
@@ -1272,6 +1275,9 @@ arch_get_unmapped_area_topdown(struct fi
 	if (len > TASK_SIZE)
 		return -ENOMEM;
 
+	if (flags & MAP_FIXED)
+		return addr;
+
 	/* requesting a specific address */
 	if (addr) {
 		addr = PAGE_ALIGN(addr);
@@ -1360,22 +1366,21 @@ get_unmapped_area(struct file *file, uns
 		unsigned long pgoff, unsigned long flags)
 {
 	unsigned long ret;
+	unsigned long (*get_area)(struct file *, unsigned long,
+				  unsigned long, unsigned long, unsigned long);
 
-	if (!(flags & MAP_FIXED)) {
-		unsigned long (*get_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
-
-		get_area = current->mm->get_unmapped_area;
-		if (file && file->f_op && file->f_op->get_unmapped_area)
-			get_area = file->f_op->get_unmapped_area;
-		addr = get_area(file, addr, len, pgoff, flags);
-		if (IS_ERR_VALUE(addr))
-			return addr;
-	}
+	get_area = current->mm->get_unmapped_area;
+	if (file && file->f_op && file->f_op->get_unmapped_area)
+		get_area = file->f_op->get_unmapped_area;
+	addr = get_area(file, addr, len, pgoff, flags);
+	if (IS_ERR_VALUE(addr))
+		return addr;
 
 	if (addr > TASK_SIZE - len)
 		return -ENOMEM;
 	if (addr & ~PAGE_MASK)
 		return -EINVAL;
+
 	if (file && is_file_hugepages(file))  {
 		/*
 		 * Check if the given range is hugepage aligned, and

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[PATCH 14/14] get_unmapped_area doesn't need hugetlbfs hacks anymore

[Benjamin Herrenschmidt]

---

 mm/mmap.c |   16 ----------------
 1 file changed, 16 deletions(-)

Index: linux-cell/mm/mmap.c
===================================================================
--- linux-cell.orig/mm/mmap.c	2007-03-22 16:30:24.000000000 +1100
+++ linux-cell/mm/mmap.c	2007-03-22 16:30:48.000000000 +1100
@@ -1381,22 +1381,6 @@ get_unmapped_area(struct file *file, uns
 	if (addr & ~PAGE_MASK)
 		return -EINVAL;
 
-	if (file && is_file_hugepages(file))  {
-		/*
-		 * Check if the given range is hugepage aligned, and
-		 * can be made suitable for hugepages.
-		 */
-		ret = prepare_hugepage_range(addr, len, pgoff);
-	} else {
-		/*
-		 * Ensure that a normal request is not falling in a
-		 * reserved hugepage range.  For some archs like IA-64,
-		 * there is a separate region for hugepages.
-		 */
-		ret = is_hugepage_only_range(current->mm, addr, len);
-	}
-	if (ret)
-		return -EINVAL;
 	return addr;
 }
 

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Re: [PATCH 0/14] Pass MAP_FIXED down to get_unmapped_area

[David Howells]

Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> This serie of patches moves the logic to handle MAP_FIXED down to the
> various arch/driver get_unmapped_area() implementations, and then changes
> the generic code to always call them. The hugetlbfs hacks then disappear
> from the generic code.

This sounds like get_unmapped_area() is now doing more than it says on the
tin.  As I understand it, it's to be called to locate an unmapped area when
one wasn't specified by MAP_FIXED, and so shouldn't be called if MAP_FIXED is
set.

Admittedly, on NOMMU, it's also used to find the location of quasi-memory
devices such as framebuffers and ramfs files, but that's not a great deviation
from the original intent.

Perhaps a change of name is in order for the function?

> Since I need to do some special 64K pages mappings for SPEs on cell, I need
> to work around the first problem at least. I have further patches thus
> implementing a "slices" layer that handles multiple page sizes through
> slices of the address space for use by hugetlbfs, the SPE code, and possibly
> others, but it requires that serie of patches first/

That makes it sound like there should be an "unget" too for when an error
occurs between ->get_unmapped_area() being called and ->mmap() returning
successfully.

David

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Re: [PATCH 0/14] Pass MAP_FIXED down to get_unmapped_area

[Benjamin Herrenschmidt]

On Wed, 2007-04-04 at 11:31 +0100, David Howells wrote:
> Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> 
> > This serie of patches moves the logic to handle MAP_FIXED down to the
> > various arch/driver get_unmapped_area() implementations, and then changes
> > the generic code to always call them. The hugetlbfs hacks then disappear
> > from the generic code.
> 
> This sounds like get_unmapped_area() is now doing more than it says on the
> tin.  As I understand it, it's to be called to locate an unmapped area when
> one wasn't specified by MAP_FIXED, and so shouldn't be called if MAP_FIXED is
> set.

Well... that was the initial implementation. But that doesn't quite deal
well with various issues like page size constraints like the segment
constraints on powerpc or other hugetlbfs realted issues, and the
aliasing problems on architectures with virtually caches...

Just look at how many architectures already have special case for
MAP_FIXED in their arch_get_unmapped_area ! It was never called so far
though, my patch makes it being called.

I agree it's probably not the best interface, but I'm still trying to
figure out something that would be nicer as a "second step", as I don't
want to do too much in one set of patches. This serie allows me to hook
in my SPE 64K page thingy, to cleanup & improve a bit my hugetlb
handling, and possibly fixes some of those aliasing issues on
architectures with virtual caches...

> Admittedly, on NOMMU, it's also used to find the location of quasi-memory
> devices such as framebuffers and ramfs files, but that's not a great deviation
> from the original intent.
> 
> Perhaps a change of name is in order for the function?

I'm not sure. "get" can mean "obtain" :-) The way it's currently
implemented for me on powerpc works fine that way, I don't need an
"unget".

> > Since I need to do some special 64K pages mappings for SPEs on cell, I need
> > to work around the first problem at least. I have further patches thus
> > implementing a "slices" layer that handles multiple page sizes through
> > slices of the address space for use by hugetlbfs, the SPE code, and possibly
> > others, but it requires that serie of patches first/
> 
> That makes it sound like there should be an "unget" too for when an error
> occurs between ->get_unmapped_area() being called and ->mmap() returning
> successfully.

I don't need it because I can flip the page size of the segment back if
it has no VMA in it on the next get_unmapped_area(). Again, I'd like to
come up with a better interface, and I might post something in that
direction next week, but I beleive those patches (+/- bug fixes) are a
good first step in the right direction. I also need to find a proper way
to solve the mremap problem as it's bogus as it is already with things
like hugetlbfs on powerpc at least. 

Ben.


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[PATCH 0/14] Pass MAP_FIXED down to get_unmapped_area

[Benjamin Herrenschmidt]

This is a "first step" as there are still cleanups to be done in various
areas touched by that code but I think it's probably good to go as is and
at least enables me to implement what I need for PowerPC.

The current get_unmapped_area code calls the f_ops->get_unmapped_area or
the arch one (via the mm) only when MAP_FIXED is not passed. That makes
it impossible for archs to impose proper constraints on regions of the
virtual address space. To work around that, get_unmapped_area() then
calls some hugetlbfs specific hacks.

This cause several problems, among others:

 - It makes it impossible for a driver or filesystem to do the same thing
that hugetlbfs does (for example, to allow a driver to use larger page
sizes to map external hardware) if that requires applying a constraint
on the addresses (constraining that mapping in certain regions and other
mappings out of those regions).

 - Some archs like arm, mips, sparc, sparc64, sh and sh64 already want
MAP_FIXED to be passed down in order to deal with aliasing issues.
The code is there to handle it... but is never called.

This serie of patches moves the logic to handle MAP_FIXED down to the
various arch/driver get_unmapped_area() implementations, and then changes
the generic code to always call them. The hugetlbfs hacks then disappear
from the generic code.

Since I need to do some special 64K pages mappings for SPEs on cell, I need
to work around the first problem at least. I have further patches thus
implementing a "slices" layer that handles multiple page sizes through
slices of the address space for use by hugetlbfs, the SPE code, and possibly
others, but it requires that serie of patches first/

There is still a potential (but not practical) issue due to the fact that
filesystems/drivers implemeting g_u_a will effectively bypass all arch
checks. This is not an issue in practice as the only filesystems/drivers
using that hook are doing so for arch specific purposes in the first place.

There is also a problem with mremap that will completely bypass all arch
checks. I'll try to address that separately, I'm not 100% certain yet how,
possibly by making it not work when the vma has a file whose f_ops has a
get_unmapped_area callback, and by making it use is_hugepage_only_range()
before expanding into a new area.

Also, I want to turn is_hugepage_only_range() into a more generic
is_normal_page_range() as that's really what it will end up meaning
when used in stack grow, brk grow and mremap.

None of the above "issues" however are introduced by this patch, they are
already there, so I think the patch can go in after review of the individual
bits by respective maintainers I suppose.

Cheers,
Ben.

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Re: [PATCH 0/14] Pass MAP_FIXED down to get_unmapped_area

[Benjamin Herrenschmidt]

On Wed, 2007-04-04 at 14:01 +1000, Benjamin Herrenschmidt wrote:
> This is a "first step" as there are still cleanups to be done in various
> areas touched by that code but I think it's probably good to go as is and
> at least enables me to implement what I need for PowerPC.


 .../...

And sorry for the double-send of some of the patches, a script hickup on
my side.

Cheers,
Ben.


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