[PATCH -tip] mm: introduce __GFP_PANIC modifier

[Cyrill Gorcunov <gorcunov@openvz.org>]

Hi,

here is an attempt to bring in __GFP_PANIC modifier.
The patch is made on top of -tip repo. I've been
trying to apply it in top of -mm tree but it
seems -tip is a bit newer, at least it already
has __GFP_BITS_SHIFT = 22 defined.

Mel, could you take a look?

For easier review -- here is what is done:
1) __GFP_PANIC introduced
2) __alloc_pages_internal now checks for this flag
   and panic if needed.

	-- Cyrill
---
From: Cyrill Gorcunov <gorcunov@openvz.org>
Subject: [PATCH -tip] mm: introduce __GFP_PANIC modifier

Sometime we need that memory obtained via kmalloc
is always granted cause if there is not enough memory
we just can't go further.

For such a case we introduce __GFP_PANIC modificator
If memory can't be granted -- we just panic and halt.

Note 1: it trigger panic only if we reach out-of-memory
situation. MAX_ORDER and SLAB built-in constants are
not covered by intent.

Note 2: __GFP_PANIC implicitly turn off failslab
facility on such kind of calls.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
---
 include/linux/gfp.h |    4 +++-
 mm/failslab.c       |    3 +++
 mm/page_alloc.c     |   10 ++++++++--
 3 files changed, 14 insertions(+), 3 deletions(-)

Index: linux-2.6.git/include/linux/gfp.h
=====================================================================
--- linux-2.6.git.orig/include/linux/gfp.h
+++ linux-2.6.git/include/linux/gfp.h
@@ -58,7 +58,9 @@ struct vm_area_struct;
 #define __GFP_NOTRACK	((__force gfp_t)0)
 #endif
 
-#define __GFP_BITS_SHIFT 22	/* Room for 22 __GFP_FOO bits */
+#define __GFP_PANIC	((__force gfp_t)0x400000u) /* Panic on page alloction failure */
+
+#define __GFP_BITS_SHIFT 23	/* Room for 23 __GFP_FOO bits */
 #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
 
 /* This equals 0, but use constants in case they ever change */
Index: linux-2.6.git/mm/failslab.c
=====================================================================
--- linux-2.6.git.orig/mm/failslab.c
+++ linux-2.6.git/mm/failslab.c
@@ -17,6 +17,9 @@ bool should_failslab(size_t size, gfp_t 
 	if (gfpflags & __GFP_NOFAIL)
 		return false;
 
+	if (gfpflags & __GFP_PANIC)
+		return false;
+
         if (failslab.ignore_gfp_wait && (gfpflags & __GFP_WAIT))
 		return false;
 
Index: linux-2.6.git/mm/page_alloc.c
=====================================================================
--- linux-2.6.git.orig/mm/page_alloc.c
+++ linux-2.6.git/mm/page_alloc.c
@@ -1185,6 +1185,8 @@ static int should_fail_alloc_page(gfp_t 
 		return 0;
 	if (gfp_mask & __GFP_NOFAIL)
 		return 0;
+	if (gfp_mask & __GFP_PANIC)
+		return 0;
 	if (fail_page_alloc.ignore_gfp_highmem && (gfp_mask & __GFP_HIGHMEM))
 		return 0;
 	if (fail_page_alloc.ignore_gfp_wait && (gfp_mask & __GFP_WAIT))
@@ -1506,7 +1508,7 @@ restart:
 		 * Happens if we have an empty zonelist as a result of
 		 * GFP_THISNODE being used on a memoryless node
 		 */
-		return NULL;
+		goto nopage;
 	}
 
 	page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask, order,
@@ -1685,7 +1687,11 @@ nopage:
 		dump_stack();
 		show_mem();
 	}
-	return page;
+	if (unlikely(gfp_mask & __GFP_PANIC))
+		panic("Out of memory: panic due to __GFP_PANIC."
+			" %s order:%d, gfp_mask:0x%x\n", p->comm,
+			order, gfp_mask);
+	return NULL;
 got_pg:
 	if (kmemcheck_enabled)
 		kmemcheck_pagealloc_alloc(page, order, gfp_mask);