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    Subject[PATCH 04/14] mm: allow PF_MEMALLOC from softirq context
    Date
    This is needed to allow network softirq packet processing to make use
    of PF_MEMALLOC.

    Currently softirq context cannot use PF_MEMALLOC due to it not being
    associated with a task, and therefore not having task flags to fiddle with -
    thus the gfp to alloc flag mapping ignores the task flags when in interrupts
    (hard or soft) context.

    Allowing softirqs to make use of PF_MEMALLOC therefore requires some trickery.
    We basically borrow the task flags from whatever process happens to be
    preempted by the softirq.

    So we modify the gfp to alloc flags mapping to not exclude task flags in
    softirq context, and modify the softirq code to save, clear and restore
    the PF_MEMALLOC flag.

    The save and clear, ensures the preempted task's PF_MEMALLOC flag doesn't
    leak into the softirq. The restore ensures a softirq's PF_MEMALLOC flag
    cannot leak back into the preempted process.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Mel Gorman <mgorman@suse.de>
    ---
    include/linux/sched.h | 7 +++++++
    kernel/softirq.c | 3 +++
    mm/page_alloc.c | 5 ++++-
    3 files changed, 14 insertions(+), 1 deletions(-)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 2a8621c..7f908f9 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -1840,6 +1840,13 @@ static inline void rcu_copy_process(struct task_struct *p)

    #endif

    +static inline void tsk_restore_flags(struct task_struct *p,
    + unsigned long pflags, unsigned long mask)
    +{
    + p->flags &= ~mask;
    + p->flags |= pflags & mask;
    +}
    +
    #ifdef CONFIG_SMP
    extern void do_set_cpus_allowed(struct task_struct *p,
    const struct cpumask *new_mask);
    diff --git a/kernel/softirq.c b/kernel/softirq.c
    index 1396017..2817c27 100644
    --- a/kernel/softirq.c
    +++ b/kernel/softirq.c
    @@ -210,6 +210,8 @@ asmlinkage void __do_softirq(void)
    __u32 pending;
    int max_restart = MAX_SOFTIRQ_RESTART;
    int cpu;
    + unsigned long pflags = current->flags;
    + current->flags &= ~PF_MEMALLOC;

    pending = local_softirq_pending();
    account_system_vtime(current);
    @@ -265,6 +267,7 @@ restart:

    account_system_vtime(current);
    __local_bh_enable(SOFTIRQ_OFFSET);
    + tsk_restore_flags(current, pflags, PF_MEMALLOC);
    }

    #ifndef __ARCH_HAS_DO_SOFTIRQ
    diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    index 27043e7..4e19606 100644
    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -2046,7 +2046,10 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
    if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
    if (gfp_mask & __GFP_MEMALLOC)
    alloc_flags |= ALLOC_NO_WATERMARKS;
    - else if (likely(!(gfp_mask & __GFP_NOMEMALLOC)) && !in_interrupt())
    + else if (!in_irq() && (current->flags & PF_MEMALLOC))
    + alloc_flags |= ALLOC_NO_WATERMARKS;
    + else if (!in_interrupt() &&
    + unlikely(test_thread_flag(TIF_MEMDIE)))
    alloc_flags |= ALLOC_NO_WATERMARKS;
    }

    --
    1.7.3.4


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