Re: page migration: Only migrate pages if allocation in the highest zone is possible

[Christoph Lameter <clameter@sgi.com>]

On Mon, 30 Apr 2007, Hugh Dickins wrote:

> That looks about right to me: from the previous discussion it appeared
> that a test in vma_migratable() is currently more appropriate than
> refining the page allocation routines (and yours is nicer than mine).
> Though that will change when page migration is used beyond CONFIG_NUMA.

One can pass a function to page migration functions. If that function
is not doing highest zone allocations based on nodes (for example to move 
pages around in a zone) then we would be fine. Thats why I said the 
"common use of page migration".

> I would feel more comfortable if you #included linux/pagemap.h
> and used the mapping_gfp_mask macro, just for documentation of
> what's going on - that's how that field is accessed elsewhere;
> though I agree that it looks like gfp_zone() will happen to
> work on the raw flags field.

Ok.

> Conversely (why do I suggest safety on one line and unsafety
> on another? incorribly inconsistent or what?) I contend that
> testing vma->vm_file->f_mapping there is unnecessary, and I'd
> rather we don't propagate unnecessary such tests (each one
> sows another seed of doubt for the next): sys_fsync looks
> like one syscall which would crash if f_mapping ever NULL.

I pawned the check off vma_wants_writenotify(). So I am already 
propagating bad habits. sigh.




page migration: Only migrate pages if allocation in the highest zone is possible

Address spaces contain an allocation flag that specifies restriction on
the zone for pages placed in the mapping. I.e. some device may require pages
to be allocated from a DMA zone. Block devices may not be able to use pages
from HIGHMEM.

Memory policies and the common use of page migration works only on the
highest zone. If the address space does not allow allocation from the
highest zone then the pages in the address space are not migratable simply
because we can only allocate memory for a specified node if we allow
allocation for the highest zone on each node.

Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/migrate.h |   11 +++++++++++
 1 file changed, 11 insertions(+)

Index: linux-2.6.21-rc7-mm2/include/linux/migrate.h
===================================================================
--- linux-2.6.21-rc7-mm2.orig/include/linux/migrate.h	2007-04-29 23:58:47.000000000 -0700
+++ linux-2.6.21-rc7-mm2/include/linux/migrate.h	2007-04-30 12:18:41.000000000 -0700
@@ -2,6 +2,8 @@
 #define _LINUX_MIGRATE_H
 
 #include <linux/mm.h>
+#include <linux/mempolicy.h>
+#include <linux/pagemap.h>
 
 typedef struct page *new_page_t(struct page *, unsigned long private, int **);
 
@@ -10,6 +12,15 @@ static inline int vma_migratable(struct 
 {
 	if (vma->vm_flags & (VM_IO|VM_HUGETLB|VM_PFNMAP|VM_RESERVED))
 		return 0;
+	/*
+	 * Migration allocates pages in the highest zone. If we cannot
+	 * do so then migration (at least from node to node) is not
+	 * possible.
+	 */
+	if (vma->vm_file &&
+		gfp_zone(mapping_gfp_mask(vma->vm_file->f_mapping))
+								< policy_zone)
+			return 0;
 	return 1;
 }
 

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