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SubjectRe: [RFC] Restrict size of page_cgroup->flags
On Wed, 6 Oct 2010 19:53:14 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> 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
>
I agree that reducing the size of page_cgroup would be good and important.
But, wouldn't it be better to remove ->page, if possible ?

Thanks,
Daisuke Nishimura.

> 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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