[RFC 1/7] mm: Add new vma flag VM_LOCAL_CPU

[Boaz Harrosh <boazh@netapp.com>]

On a call to mmap an mmap provider (like an FS) can put
this flag on vma->vm_flags.

This tells the Kernel that the vma will be used from a single
core only and therefore invalidation of PTE(s) need not a
wide CPU scheduling

The motivation of this flag is the ZUFS project where we want
to optimally map user-application buffers into a user-mode-server
execute the operation and efficiently unmap.

In this project we utilize a per-core server thread so everything
is kept local. If we use the regular zap_ptes() API All CPU's
are scheduled for the unmap, though in our case we know that we
have only used a single core. The regular zap_ptes adds a very big
latency on every operations and mostly kills the concurrency of the
over all system. Because it imposes a serialization between all cores

Some preliminary measurements on a 40 core machines:

	unpatched		patched
Threads	Op/s	Lat [us]	Op/s	Lat [us]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1	185391	4.9		200799	4.6
2	197993	9.6		314321	5.9
4	310597	12.1		565574	6.6
8	546702	13.8		1113138	6.6
12	641728	17.2		1598451	6.8
18	744750	22.2		1648689	7.8
24	790805	28.3		1702285	8
36	849763	38.9		1783346	13.4
48	792000	44.6		1741873	17.4

[FIXME]
	We need to actually impose this policy. On very first
	pte_insert we should sample the used CPU_ID and on all
	susequent pte_inserts we need to make sure it is the
	same CPU_ID used.

NOTE: That this vma is never used during a page_fault. It is
always used in a synchronous way from an affinity set thread
to a single core.

Signed-off-by: Boaz Harrosh <boazh@netapp.com>
---
 fs/proc/task_mmu.c | 3 +++
 include/linux/mm.h | 3 +++
 mm/memory.c        | 2 +-
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 339e4c1..20786ba 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -681,6 +681,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 		[ilog2(VM_PKEY_BIT2)]	= "",
 		[ilog2(VM_PKEY_BIT3)]	= "",
 #endif
+#ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS
+		[ilog2(VM_LOCAL_CPU)]	= "lc",
+#endif
 	};
 	size_t i;
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ea818ff..02bb8b5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -226,6 +226,9 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_HIGH_ARCH_2	BIT(VM_HIGH_ARCH_BIT_2)
 #define VM_HIGH_ARCH_3	BIT(VM_HIGH_ARCH_BIT_3)
 #define VM_HIGH_ARCH_4	BIT(VM_HIGH_ARCH_BIT_4)
+#define VM_LOCAL_CPU	BIT(37)		/* FIXME: Needs to move from here */
+#else /* ! CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
+#define VM_LOCAL_CPU	0		/* FIXME: Needs to move from here */
 #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
 
 #if defined(CONFIG_X86)
diff --git a/mm/memory.c b/mm/memory.c
index 7930046..7620ced 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1804,7 +1804,7 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
 				goto out_unlock;
 			entry = *pte;
 			goto out_mkwrite;
-		} else
+		} else if (!(vma->vm_flags & VM_LOCAL_CPU))
 			goto out_unlock;
 	}
 
-- 
2.5.5