[RFC] Restrict size of page_cgroup->flags

[Balbir Singh <balbir@linux.vnet.ibm.com>]

I propose restricting page_cgroup.flags to 16 bits. The patch for the
same is below. Comments?


Restrict the bits usage in page_cgroup.flags

From: Balbir Singh <balbir@linux.vnet.ibm.com>

Restricting the flags helps control growth of the flags unbound.
Restriciting it to 16 bits gives us the possibility of merging
cgroup id with flags (atomicity permitting) and saving a whole
long word in page_cgroup

Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
---

 include/linux/page_cgroup.h |    3 +++
 mm/page_cgroup.c            |    1 +
 2 files changed, 4 insertions(+), 0 deletions(-)


diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index 872f6b1..10c37b4 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -44,8 +44,11 @@ enum {
 	PCG_FILE_WRITEBACK, /* page is under writeback */
 	PCG_FILE_UNSTABLE_NFS, /* page is NFS unstable */
 	PCG_MIGRATION, /* under page migration */
+	PCG_MAX_NR,
 };
 
+#define PCG_MAX_BIT_SIZE	16
+
 #define TESTPCGFLAG(uname, lname)			\
 static inline int PageCgroup##uname(struct page_cgroup *pc)	\
 	{ return test_bit(PCG_##lname, &pc->flags); }
diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 5bffada..e16ad2e 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -258,6 +258,7 @@ void __init page_cgroup_init(void)
 	unsigned long pfn;
 	int fail = 0;
 
+	BUILD_BUG_ON(PCG_MAX_NR >= PCG_MAX_BIT_SIZE);
 	if (mem_cgroup_disabled())
 		return;
 

-- 
	Three Cheers,
	Balbir

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