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    SubjectRe: [PATCH] mm/hugetlb_cgroup: Add list_del to remove unused page from hugepage_activelist when hugepage migration
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    Wanpeng Li <liwp.linux@gmail.com> writes:

    > On Wed, Jul 11, 2012 at 02:02:23PM +0530, Aneesh Kumar K.V wrote:
    >>Wanpeng Li <liwp.linux@gmail.com> writes:
    >>
    >>> From: Wanpeng Li <liwp@linux.vnet.ibm.com>
    >>>
    >>> hugepage_activelist is used to track currently used HugeTLB pages.
    >>> We can find the in-use HugeTLB pages to support HugeTLB cgroup
    >>> removal. Don't keep unused page in hugetlb_activelist too long.
    >>> Otherwise, on cgroup removal we should update the unused page's
    >>> charge to parent count. To reduce this overhead, remove unused page
    >>> from hugepage_activelist immediately.
    >>>
    >>> Signed-off-by: Wanpeng Li <liwp.linux@gmail.com>
    >>> ---
    >>> mm/hugetlb_cgroup.c | 1 +
    >>> 1 files changed, 1 insertions(+), 0 deletions(-)
    >>>
    >>> diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
    >>> index b834e8d..d819d66 100644
    >>> --- a/mm/hugetlb_cgroup.c
    >>> +++ b/mm/hugetlb_cgroup.c
    >>> @@ -398,6 +398,7 @@ void hugetlb_cgroup_migrate(struct page *oldhpage, struct page *newhpage)
    >>> spin_lock(&hugetlb_lock);
    >>> h_cg = hugetlb_cgroup_from_page(oldhpage);
    >>> set_hugetlb_cgroup(oldhpage, NULL);
    >>> + list_del(&oldhpage->lru);
    >>>
    >>> /* move the h_cg details to new cgroup */
    >>> set_hugetlb_cgroup(newhpage, h_cg);
    >>
    >>put_page on the oldhpage will do that. If we do list_del here
    >>free_huge_page() will have error list_move().
    >
    > I don't know if move the page to other lists when migration in order
    > to reduce overhead I mentioned make sense. :-)
    >

    Even if we are in the active_list, we don't get anything wrong because
    hugetlb cgroup is set to NULL for the old hugetlb page. The above patch is
    wrong and will result in crash when we try to put_page the oldhpage.

    -aneesh



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