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    SubjectRe: [PATCH 3/5] writeback: fix dirtied pages accounting on sub-page writes
    On Tue 22-11-11 20:30:01, Wu Fengguang wrote:
    > > > @@ -1743,6 +1738,8 @@ void account_page_dirtied(struct page *p
    > > > __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
    > > > task_dirty_inc(current);
    > > > task_io_account_write(PAGE_CACHE_SIZE);
    > > > + current->nr_dirtied++;
    > > > + __get_cpu_var(bdp_ratelimits)++;
    > > I think you need preempt_disable() and preempt_enable() pair around
    > > __get_cpu_var(). Otherwise a process could get rescheduled in the middle of
    > > read-modify-write cycle...
    >
    > Hmm, I'm not worried about it at all, because bdp_ratelimits don't
    > need to be accurate. In normal cases it won't even trigger one single
    > call to balance_dirty_pages().
    I agree regarding the accuracy. But the CPU can change when the process
    is scheduled again. So you could modify counter of a CPU you are not
    running on. And that can cause bad things...

    > btw, account_page_dirtied() is called inside spinlock, will it be
    > sufficient?
    Currently it is not enough in real-time kernels and when sleeping
    spinlocks work gets merged it won't be enough even in standard kernels...
    And in kernels where spinlock means preemption is disabled
    preempt_enable/disable will be almost for free...

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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