[PATCHv3 0/2] mm: infrastructure for correctly handling foreign pages on Xen

[PATCHv3 0/2] mm: infrastructure for correctly handling foreign pages on Xen

[David Vrabel]

These two patches are the common parts of a larger Xen series[1]
fixing several long-standing bugs the handling of foreign[2] pages in
Xen guests.

Andrew, these are best merged via the Xen tree.  Can I have an acked-by?

The first patch is required to fix get_user_pages[_fast]() with
userspace space mappings of such foreign pages.  Basically, pte_page()
doesn't work so an alternate mechanism is needed to get the page from
a VMA and address.  By requiring mappings needing this method are
'special' this should not have an impact on the common use cases.

The second patch isn't essential but helps with readability of the
resulting user of the page flag.

For further background reading see:

  http://xenbits.xen.org/people/dvrabel/grant-improvements-C.pdf

Changes in v3:

- find_page renamed to find_special_page.
- added documentation.

Changes in v2:

- Add a find_page VMA op instead of the pages field so: a) the size of
  struct vm_area_struct does not increase; and b) the common code need
  not handling splitting the pages area.

David

[1] http://lists.xen.org/archives/html/xen-devel/2015-01/msg00979.html

[2] Another guest's page temporarily granted to this guest.

[PATCHv3 0/2] mm: infrastructure for correctly handling foreign pages on Xen

[David Vrabel]

These two patches are the common parts of a larger Xen series[1]
fixing several long-standing bugs the handling of foreign[2] pages in
Xen guests.

Andrew, these are best merged via the Xen tree.  Can I have an acked-by?

The first patch is required to fix get_user_pages[_fast]() with
userspace space mappings of such foreign pages.  Basically, pte_page()
doesn't work so an alternate mechanism is needed to get the page from
a VMA and address.  By requiring mappings needing this method are
'special' this should not have an impact on the common use cases.

The second patch isn't essential but helps with readability of the
resulting user of the page flag.

For further background reading see:

  http://xenbits.xen.org/people/dvrabel/grant-improvements-C.pdf

Changes in v3:

- find_page renamed to find_special_page.
- added documentation.

Changes in v2:

- Add a find_page VMA op instead of the pages field so: a) the size of
  struct vm_area_struct does not increase; and b) the common code need
  not handling splitting the pages area.

David

[1] http://lists.xen.org/archives/html/xen-devel/2015-01/msg00979.html

[2] Another guest's page temporarily granted to this guest.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

[PATCH 1/2] mm: provide a find_special_page vma operation

[David Vrabel]

The optional find_special_page VMA operation is used to lookup the
pages backing a VMA.  This is useful in cases where the normal
mechanisms for finding the page don't work.  This is only called if
the PTE is special.

One use case is a Xen PV guest mapping foreign pages into userspace.

In a Xen PV guest, the PTEs contain MFNs so get_user_pages() (for
example) must do an MFN to PFN (M2P) lookup before it can get the
page.  For foreign pages (those owned by another guest) the M2P lookup
returns the PFN as seen by the foreign guest (which would be
completely the wrong page for the local guest).

This cannot be fixed up improving the M2P lookup since one MFN may be
mapped onto two or more pages so getting the right page is impossible
given just the MFN.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 include/linux/mm.h |    8 ++++++++
 mm/memory.c        |    2 ++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 80fc92a..9269af7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -290,6 +290,14 @@ struct vm_operations_struct {
 	/* called by sys_remap_file_pages() to populate non-linear mapping */
 	int (*remap_pages)(struct vm_area_struct *vma, unsigned long addr,
 			   unsigned long size, pgoff_t pgoff);
+
+	/*
+	 * Called by vm_normal_page() for special PTEs to find the
+	 * page for @addr.  This is useful if the default behavior
+	 * (using pte_page()) would not find the correct page.
+	 */
+	struct page *(*find_special_page)(struct vm_area_struct *vma,
+					  unsigned long addr);
 };
 
 struct mmu_gather;
diff --git a/mm/memory.c b/mm/memory.c
index 54f3a9b..dc2e01a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -754,6 +754,8 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 	if (HAVE_PTE_SPECIAL) {
 		if (likely(!pte_special(pte)))
 			goto check_pfn;
+		if (vma->vm_ops && vma->vm_ops->find_special_page)
+			return vma->vm_ops->find_special_page(vma, addr);
 		if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
 			return NULL;
 		if (!is_zero_pfn(pfn))
-- 
1.7.10.4

[PATCH 1/2] mm: provide a find_special_page vma operation

[David Vrabel]

The optional find_special_page VMA operation is used to lookup the
pages backing a VMA.  This is useful in cases where the normal
mechanisms for finding the page don't work.  This is only called if
the PTE is special.

One use case is a Xen PV guest mapping foreign pages into userspace.

In a Xen PV guest, the PTEs contain MFNs so get_user_pages() (for
example) must do an MFN to PFN (M2P) lookup before it can get the
page.  For foreign pages (those owned by another guest) the M2P lookup
returns the PFN as seen by the foreign guest (which would be
completely the wrong page for the local guest).

This cannot be fixed up improving the M2P lookup since one MFN may be
mapped onto two or more pages so getting the right page is impossible
given just the MFN.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 include/linux/mm.h |    8 ++++++++
 mm/memory.c        |    2 ++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 80fc92a..9269af7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -290,6 +290,14 @@ struct vm_operations_struct {
 	/* called by sys_remap_file_pages() to populate non-linear mapping */
 	int (*remap_pages)(struct vm_area_struct *vma, unsigned long addr,
 			   unsigned long size, pgoff_t pgoff);
+
+	/*
+	 * Called by vm_normal_page() for special PTEs to find the
+	 * page for @addr.  This is useful if the default behavior
+	 * (using pte_page()) would not find the correct page.
+	 */
+	struct page *(*find_special_page)(struct vm_area_struct *vma,
+					  unsigned long addr);
 };
 
 struct mmu_gather;
diff --git a/mm/memory.c b/mm/memory.c
index 54f3a9b..dc2e01a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -754,6 +754,8 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 	if (HAVE_PTE_SPECIAL) {
 		if (likely(!pte_special(pte)))
 			goto check_pfn;
+		if (vma->vm_ops && vma->vm_ops->find_special_page)
+			return vma->vm_ops->find_special_page(vma, addr);
 		if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
 			return NULL;
 		if (!is_zero_pfn(pfn))
-- 
1.7.10.4

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

[PATCH 2/2] mm: add 'foreign' alias for the 'pinned' page flag

[David Vrabel]

From: Jenny Herbert <jennifer.herbert@citrix.com>

The foreign page flag will be used by Xen guests to mark pages that
have grant mappings of frames from other (foreign) guests.

The foreign flag is an alias for the existing (Xen-specific) pinned
flag.  This is safe because pinned is only used on pages used for page
tables and these cannot also be foreign.

Signed-off-by: Jenny Herbert <jennifer.herbert@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 include/linux/page-flags.h |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index e1f5fcd..5ed7bda 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -121,8 +121,12 @@ enum pageflags {
 	PG_fscache = PG_private_2,	/* page backed by cache */
 
 	/* XEN */
+	/* Pinned in Xen as a read-only pagetable page. */
 	PG_pinned = PG_owner_priv_1,
+	/* Pinned as part of domain save (see xen_mm_pin_all()). */
 	PG_savepinned = PG_dirty,
+	/* Has a grant mapping of another (foreign) domain's page. */
+	PG_foreign = PG_owner_priv_1,
 
 	/* SLOB */
 	PG_slob_free = PG_private,
@@ -215,6 +219,7 @@ __PAGEFLAG(Slab, slab)
 PAGEFLAG(Checked, checked)		/* Used by some filesystems */
 PAGEFLAG(Pinned, pinned) TESTSCFLAG(Pinned, pinned)	/* Xen */
 PAGEFLAG(SavePinned, savepinned);			/* Xen */
+PAGEFLAG(Foreign, foreign);				/* Xen */
 PAGEFLAG(Reserved, reserved) __CLEARPAGEFLAG(Reserved, reserved)
 PAGEFLAG(SwapBacked, swapbacked) __CLEARPAGEFLAG(SwapBacked, swapbacked)
 	__SETPAGEFLAG(SwapBacked, swapbacked)
-- 
1.7.10.4

[PATCH 2/2] mm: add 'foreign' alias for the 'pinned' page flag

[David Vrabel]

From: Jenny Herbert <jennifer.herbert@citrix.com>

The foreign page flag will be used by Xen guests to mark pages that
have grant mappings of frames from other (foreign) guests.

The foreign flag is an alias for the existing (Xen-specific) pinned
flag.  This is safe because pinned is only used on pages used for page
tables and these cannot also be foreign.

Signed-off-by: Jenny Herbert <jennifer.herbert@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 include/linux/page-flags.h |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index e1f5fcd..5ed7bda 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -121,8 +121,12 @@ enum pageflags {
 	PG_fscache = PG_private_2,	/* page backed by cache */
 
 	/* XEN */
+	/* Pinned in Xen as a read-only pagetable page. */
 	PG_pinned = PG_owner_priv_1,
+	/* Pinned as part of domain save (see xen_mm_pin_all()). */
 	PG_savepinned = PG_dirty,
+	/* Has a grant mapping of another (foreign) domain's page. */
+	PG_foreign = PG_owner_priv_1,
 
 	/* SLOB */
 	PG_slob_free = PG_private,
@@ -215,6 +219,7 @@ __PAGEFLAG(Slab, slab)
 PAGEFLAG(Checked, checked)		/* Used by some filesystems */
 PAGEFLAG(Pinned, pinned) TESTSCFLAG(Pinned, pinned)	/* Xen */
 PAGEFLAG(SavePinned, savepinned);			/* Xen */
+PAGEFLAG(Foreign, foreign);				/* Xen */
 PAGEFLAG(Reserved, reserved) __CLEARPAGEFLAG(Reserved, reserved)
 PAGEFLAG(SwapBacked, swapbacked) __CLEARPAGEFLAG(SwapBacked, swapbacked)
 	__SETPAGEFLAG(SwapBacked, swapbacked)
-- 
1.7.10.4

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH 1/2] mm: provide a find_special_page vma operation

[Andrew Morton]

On Mon, 19 Jan 2015 15:47:22 +0000 David Vrabel <david.vrabel@citrix.com> wrote:

> The optional find_special_page VMA operation is used to lookup the
> pages backing a VMA.  This is useful in cases where the normal
> mechanisms for finding the page don't work.  This is only called if
> the PTE is special.
> 
> One use case is a Xen PV guest mapping foreign pages into userspace.
> 
> In a Xen PV guest, the PTEs contain MFNs so get_user_pages() (for
> example) must do an MFN to PFN (M2P) lookup before it can get the
> page.  For foreign pages (those owned by another guest) the M2P lookup
> returns the PFN as seen by the foreign guest (which would be
> completely the wrong page for the local guest).
> 
> This cannot be fixed up improving the M2P lookup since one MFN may be
> mapped onto two or more pages so getting the right page is impossible
> given just the MFN.

Acked-by: Andrew Morton <akpm@linux-foundation.org>

Re: [PATCH 1/2] mm: provide a find_special_page vma operation

[Andrew Morton]

On Mon, 19 Jan 2015 15:47:22 +0000 David Vrabel <david.vrabel@citrix.com> wrote:

> The optional find_special_page VMA operation is used to lookup the
> pages backing a VMA.  This is useful in cases where the normal
> mechanisms for finding the page don't work.  This is only called if
> the PTE is special.
> 
> One use case is a Xen PV guest mapping foreign pages into userspace.
> 
> In a Xen PV guest, the PTEs contain MFNs so get_user_pages() (for
> example) must do an MFN to PFN (M2P) lookup before it can get the
> page.  For foreign pages (those owned by another guest) the M2P lookup
> returns the PFN as seen by the foreign guest (which would be
> completely the wrong page for the local guest).
> 
> This cannot be fixed up improving the M2P lookup since one MFN may be
> mapped onto two or more pages so getting the right page is impossible
> given just the MFN.

Acked-by: Andrew Morton <akpm@linux-foundation.org>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH 2/2] mm: add 'foreign' alias for the 'pinned' page flag

[Andrew Morton]

On Mon, 19 Jan 2015 15:47:23 +0000 David Vrabel <david.vrabel@citrix.com> wrote:

> From: Jenny Herbert <jennifer.herbert@citrix.com>
> 
> The foreign page flag will be used by Xen guests to mark pages that
> have grant mappings of frames from other (foreign) guests.
> 
> The foreign flag is an alias for the existing (Xen-specific) pinned
> flag.  This is safe because pinned is only used on pages used for page
> tables and these cannot also be foreign.
> 

Acked-by: Andrew Morton <akpm@linux-foundation.org>

Re: [PATCH 2/2] mm: add 'foreign' alias for the 'pinned' page flag

[Andrew Morton]

On Mon, 19 Jan 2015 15:47:23 +0000 David Vrabel <david.vrabel@citrix.com> wrote:

> From: Jenny Herbert <jennifer.herbert@citrix.com>
> 
> The foreign page flag will be used by Xen guests to mark pages that
> have grant mappings of frames from other (foreign) guests.
> 
> The foreign flag is an alias for the existing (Xen-specific) pinned
> flag.  This is safe because pinned is only used on pages used for page
> tables and these cannot also be foreign.
> 

Acked-by: Andrew Morton <akpm@linux-foundation.org>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>