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authorPeter Zijlstra <peterz@infradead.org>2016-03-22 21:42:53 +0100
committerMartin Schwidefsky <schwidefsky@de.ibm.com>2016-04-15 18:16:37 +0200
commit0227f7c42d9e01b00ea8cbd635aaf92a09b54abc (patch)
tree7b796ca59c6d703b857475cab4e07595cc2dd15c /arch/s390/mm
parents390/dasd: add query host access to volume support (diff)
downloadlinux-0227f7c42d9e01b00ea8cbd635aaf92a09b54abc.tar.xz
linux-0227f7c42d9e01b00ea8cbd635aaf92a09b54abc.zip
s390: Clarify pagefault interrupt
While looking at set_task_state() users I stumbled over the s390 pfault interrupt code. Since Heiko provided a great explanation on how it worked, I figured we ought to preserve this. Also make a few little tweaks to the code to aid in readability and explicitly comment the unusual blocking scheme. Based-on-text-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Diffstat (limited to 'arch/s390/mm')
-rw-r--r--arch/s390/mm/fault.c41
1 files changed, 33 insertions, 8 deletions
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index cce577feab1e..7a3144017301 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -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);
}
}