s390: Clarify pagefault interrupt (0227f7c4) · Commits · 戴 / test

arch/s390/mm/fault.c

+33 −8

Original line number	Diff line number	Diff line
		@@ -631,6 +631,29 @@ void pfault_fini(void)
		static DEFINE_SPINLOCK(pfault_lock);
		static LIST_HEAD(pfault_list);

		#define PF_COMPLETE 0x0080

		/*
		* The mechanism of our pfault code: if Linux is running as guest, runs a user
		* space process and the user space process accesses a page that the host has
		* paged out we get a pfault interrupt.
		*
		* This allows us, within the guest, to schedule a different process. Without
		* this mechanism the host would have to suspend the whole virtual cpu until
		* the page has been paged in.
		*
		* So when we get such an interrupt then we set the state of the current task
		* to uninterruptible and also set the need_resched flag. Both happens within
		* interrupt context(!). If we later on want to return to user space we
		* recognize the need_resched flag and then call schedule(). It's not very
		* obvious how this works...
		*
		* Of course we have a lot of additional fun with the completion interrupt (->
		* host signals that a page of a process has been paged in and the process can
		* continue to run). This interrupt can arrive on any cpu and, since we have
		* virtual cpus, actually appear before the interrupt that signals that a page
		* is missing.
		*/
		static void pfault_interrupt(struct ext_code ext_code,
		unsigned int param32, unsigned long param64)
		{
		@@ -639,10 +662,9 @@ static void pfault_interrupt(struct ext_code ext_code,
		pid_t pid;

		/*
		* Get the external interruption subcode & pfault
		* initial/completion signal bit. VM stores this
		* in the 'cpu address' field associated with the
		* external interrupt.
		* Get the external interruption subcode & pfault initial/completion
		* signal bit. VM stores this in the 'cpu address' field associated
		* with the external interrupt.
		*/
		subcode = ext_code.subcode;
		if ((subcode & 0xff00) != __SUBCODE_MASK)
		@@ -658,7 +680,7 @@ static void pfault_interrupt(struct ext_code ext_code,
		if (!tsk)
		return;
		spin_lock(&pfault_lock);
		if (subcode & 0x0080) {
		if (subcode & PF_COMPLETE) {
		/* signal bit is set -> a page has been swapped in by VM */
		if (tsk->thread.pfault_wait == 1) {
		/* Initial interrupt was faster than the completion
		@@ -687,8 +709,7 @@ static void pfault_interrupt(struct ext_code ext_code,
		goto out;
		if (tsk->thread.pfault_wait == 1) {
		/* Already on the list with a reference: put to sleep */
		__set_task_state(tsk, TASK_UNINTERRUPTIBLE);
		set_tsk_need_resched(tsk);
		goto block;
		} else if (tsk->thread.pfault_wait == -1) {
		/* Completion interrupt was faster than the initial
		* interrupt (pfault_wait == -1). Set pfault_wait
		@@ -703,7 +724,11 @@ static void pfault_interrupt(struct ext_code ext_code,
		get_task_struct(tsk);
		tsk->thread.pfault_wait = 1;
		list_add(&tsk->thread.list, &pfault_list);
		__set_task_state(tsk, TASK_UNINTERRUPTIBLE);
		block:
		/* Since this must be a userspace fault, there
		* is no kernel task state to trample. Rely on the
		* return to userspace schedule() to block. */
		__set_current_state(TASK_UNINTERRUPTIBLE);
		set_tsk_need_resched(tsk);
		}
		}

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