Re: RRe: [patch 6/6] mm: fix pagecache write deadlocks

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Sun, 2006-10-15 at 17:57 +0200, Nick Piggin wrote:
> On Sun, Oct 15, 2006 at 05:47:03PM +0200, Peter Zijlstra wrote:
> > > 
> > > And we should really decouple it from preempt entirely, in case we
> > > ever want to check for it some other way in the pagefault handler.
> > 
> > How about we make sure all kmap_atomic implementations behave properly 
> > and make in_atomic true.
> 
> Hmm, but you may not be doing a copy*user within the kmap. And you may
> want an atomic copy*user not within a kmap (maybe).
> 
> I think it really would be more logical to do it in a wrapper function
> pagefault_disable() pagefault_enable()? ;)

I did that one first, but then noticed that most non trivial kmap_atomic
implementations already did the inc_preempt_count()/dec_preempt_count()
thing (except frv which did preempt_disable()/preempt_enable() ?)

Anyway, here goes:

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 mm/filemap.c |    4 +---
 mm/filemap.h |   20 ++++++++++++++++++++
 2 files changed, 21 insertions(+), 3 deletions(-)

Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c	2006-10-14 20:20:20.000000000 +0200
+++ linux-2.6/mm/filemap.c	2006-10-15 17:16:59.000000000 +0200
@@ -2140,9 +2140,8 @@ retry_noprogress:
 		 * the page lock, so we might recursively deadlock on the same
 		 * lock, or get an ABBA deadlock against a different lock, or
 		 * against the mmap_sem (which nests outside the page lock).
-		 * So increment preempt count, and use _atomic usercopies.
+		 * So use _atomic usercopies.
 		 */
-		inc_preempt_count();
 		if (likely(nr_segs == 1))
 			copied = filemap_copy_from_user_atomic(page, offset,
 							buf, bytes);
@@ -2150,7 +2149,6 @@ retry_noprogress:
 			copied = filemap_copy_from_user_iovec_atomic(page,
 						offset, cur_iov, iov_offset,
 						bytes);
-		dec_preempt_count();
 
 		if (!PageUptodate(page)) {
 			/*
Index: linux-2.6/mm/filemap.h
===================================================================
--- linux-2.6.orig/mm/filemap.h	2006-10-14 20:20:20.000000000 +0200
+++ linux-2.6/mm/filemap.h	2006-10-15 17:17:45.000000000 +0200
@@ -21,6 +21,22 @@ __filemap_copy_from_user_iovec_inatomic(
 					size_t bytes);
 
 /*
+ * By increasing the preempt_count we make sure the arch preempt
+ * handler bails out early, before taking any locks, so that the copy
+ * operation gets terminated early.
+ */
+pagefault_static inline void disable(void)
+{
+	inc_preempt_count();
+}
+
+pagefault_static inline void enable(void)
+{
+	dec_preempt_count();
+	preempt_check_resched();
+}
+
+/*
  * Copy as much as we can into the page and return the number of bytes which
  * were sucessfully copied.  If a fault is encountered then return the number of
  * bytes which were copied.
@@ -40,9 +56,11 @@ filemap_copy_from_user_atomic(struct pag
 	char *kaddr;
 	int left;
 
+	pagefault_disable();
 	kaddr = kmap_atomic(page, KM_USER0);
 	left = __copy_from_user_inatomic_nocache(kaddr + offset, buf, bytes);
 	kunmap_atomic(kaddr, KM_USER0);
+	pagefault_enable();
 
 	return bytes - left;
 }
@@ -75,10 +93,12 @@ filemap_copy_from_user_iovec_atomic(stru
 	char *kaddr;
 	size_t copied;
 
+	pagefault_disable();
 	kaddr = kmap_atomic(page, KM_USER0);
 	copied = __filemap_copy_from_user_iovec_inatomic(kaddr + offset, iov,
 							 base, bytes);
 	kunmap_atomic(kaddr, KM_USER0);
+	pagefault_enable();
 	return copied;
 }