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    Subject[PATCH] mm: Do not keep page locked during page fault while charging it for memcg
    Currently we are keeping faulted page locked throughout whole __do_fault
    call (except for page_mkwrite code path). If we do early COW we allocate a
    new page which has to be charged for a memcg (mem_cgroup_newpage_charge).
    This function, however, might block for unbounded amount of time if memcg
    oom killer is disabled because the only way out of the OOM situation is
    either an external event (kill a process from the group or resize the group
    hard limit) or internal event (that would get us under the limit). Many
    times the external event is the only chance to move forward, though.
    In the end we are keeping the faulted page locked and blocking other
    processes from faulting it in which is not good at all because we are
    basically punishing potentially an unrelated process for OOM condition
    in a different group (I have seen stuck system because of ld-2.11.1.so being
    locked).

    Let's unlock the faulted page while we are charging a new page and then
    recheck whether it wasn't truncated in the mean time. We should retry the
    fault in that case.

    Signed-off-by: Michal Hocko <mhocko@suse.cz>
    ---
    mm/memory.c | 18 +++++++++++++++++-
    1 files changed, 17 insertions(+), 1 deletions(-)

    diff --git a/mm/memory.c b/mm/memory.c
    index 87d9353..12e7ccc 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -3177,7 +3177,23 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
    ret = VM_FAULT_OOM;
    goto out;
    }
    - if (mem_cgroup_newpage_charge(page, mm, GFP_KERNEL)) {
    +
    + /* We have to drop the page lock here because memcg
    + * charging might block for unbound time if memcg oom
    + * killer is disabled.
    + */
    + unlock_page(vmf.page);
    + ret = mem_cgroup_newpage_charge(page, mm, GFP_KERNEL);
    + lock_page(vmf.page);
    +
    + if (!vmf.page->mapping) {
    + if (!ret)
    + mem_cgroup_uncharge_page(page);
    + page_cache_release(page);
    + ret = 0; /* retry the fault */
    + goto out;
    + }
    + if (ret) {
    ret = VM_FAULT_OOM;
    page_cache_release(page);
    goto out;
    --
    1.7.5.4
    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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