Re: [RFC] page fault retry with NOPAGE_RETRY

Benjamin Herrenschmidt
2006-09-23T22:35:36Z

> Perhaps we should concentrate on that for now. Did we have a patch to look
> at?

Only a hand written proto-patch. Below is a real (but untested) one.
Note that there might be still issues when called from get_user_pages()
which of course won't go back to userland. For the two usage scenario I
have in mind, it should be ok though. One (a signal pending) will loop
back in until the resource is available, the other (no_page() inserts
the PTE itself) is just fine. For the former case, I've added a
cond_resched() to the loop, we might want to look into adding the info
of wether we are coming from get_user_pages() vs. do_page_fault() to
these new arguments you want to add to page fault handlers. That would
allow in our case to do a non-interruptibe sleep when caused by
get_user_pages().

---

Add a way for a no_page() handler to request a retry of the faulting
instruction. It goes back to userland on page faults and just tries
again in get_user_pages(). I added a cond_resched() in the loop in that
later case.

Signed-off-by: Benjamin Herrenchmidt <benh@kernel.crashing.org>

Index: linux-work/include/linux/mm.h
===================================================================
--- linux-work.orig/include/linux/mm.h 2006-08-30 08:51:21.000000000 +1000
+++ linux-work/include/linux/mm.h 2006-09-24 08:25:33.000000000 +1000
@@ -623,6 +623,7 @@
*/
#define NOPAGE_SIGBUS (NULL)
#define NOPAGE_OOM ((struct page *) (-1))
+#define NOPAGE_RETRY ((struct page *) (-2))

/*
* Different kinds of faults, as returned by handle_mm_fault().
Index: linux-work/mm/memory.c
===================================================================
--- linux-work.orig/mm/memory.c 2006-08-17 16:16:06.000000000 +1000
+++ linux-work/mm/memory.c 2006-09-24 08:34:09.000000000 +1000
@@ -1081,6 +1081,7 @@
default:
BUG();
}
+ cond_resched();
}
if (pages) {
pages[i] = page;
@@ -2117,11 +2118,13 @@
* after the next truncate_count read.
*/

- /* no page was available -- either SIGBUS or OOM */
- if (new_page == NOPAGE_SIGBUS)
+ /* no page was available -- either SIGBUS, OOM or RETRY */
+ if (unlikely(new_page == NOPAGE_SIGBUS))
return VM_FAULT_SIGBUS;
- if (new_page == NOPAGE_OOM)
+ else if (unlikely(new_page == NOPAGE_OOM))
return VM_FAULT_OOM;
+ else if (unlikely(new_page == NOPAGE_RETRY))
+ return VM_FAULT_MINOR;

/*
* Should we do an early C-O-W break?