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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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    >


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