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SubjectRe: [PATCH V3] KSM: allow dedup all tasks memory
So,

> …snip…
> +static int ksm_seeker_thread(void *nothing)
> +{
> + pid_t last_pid = 1;
> + pid_t curr_pid;
> + struct task_struct *task;
> +
> + set_freezable();
> + set_user_nice(current, 5);
> +
> + while (!kthread_should_stop()) {
> + wait_while_offlining();
> +
> + try_to_freeze();
> +
> + if (!ksm_mode_always()) {
> + wait_event_freezable(ksm_seeker_thread_wait,
> + ksm_mode_always() || kthread_should_stop());
> + continue;
> + }
> +
> + /*
> + * import one task's vma per run
> + */
> + read_lock(&tasklist_lock);
> +
> + /* Try always get next task */
> + for_each_process(task) {
> + curr_pid = task_pid_nr(task);
> + if (curr_pid == last_pid) {
> + task = next_task(task);
> + break;
> + }
> +
> + if (curr_pid > last_pid)
> + break;
> + }
> +
> + last_pid = task_pid_nr(task);
> + ksm_import_task_vma(task);

This seems to be a bad idea. ksm_import_task_vma() may sleep with
tasklist_lock being held. Thus, IIUC, you'll get this:

[ 1754.410322] BUG: scheduling while atomic: ksmd_seeker/50/0x00000002

[ 1754.410444] Call Trace:
[ 1754.410455] dump_stack+0x5c/0x80
[ 1754.410460] __schedule_bug.cold.19+0x38/0x51
[ 1754.410464] __schedule+0x11dc/0x2080
[ 1754.410483] schedule+0x32/0xb0
[ 1754.410487] rwsem_down_write_failed+0x15d/0x240
[ 1754.410496] call_rwsem_down_write_failed+0x13/0x20
[ 1754.410499] down_write+0x20/0x30
[ 1754.410502] ksm_import_task_vma+0x22/0x70
[ 1754.410505] ksm_seeker_thread+0x134/0x1c0
[ 1754.410512] kthread+0x113/0x130
[ 1754.410518] ret_from_fork+0x35/0x40

I think you may want to get a reference to task_struct before releasing
tasklist_lock, and then put it after ksm_import_task_vma() does its job.

> + read_unlock(&tasklist_lock);
> +
> + schedule_timeout_interruptible(
> + msecs_to_jiffies(ksm_thread_seeker_sleep_millisecs));
> + }
> + return 0;
> +}
> …snip…

--
Oleksandr Natalenko (post-factum)

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