Subject: [PATCH 31/40] mm: balance_dirty_pages() vs throttle_vm_writeout() deadlock

From: Peter Zijlstra <[email-masked]>
Date: Fri, 04 May 2007 12:27:22 +0200

If we have a lot of dirty memory and hit the throttle in balance_dirty_pages()
we (potentially) generate a lot of writeback and unstable pages, if however
during this writeback we need to reclaim a bit, we might hit
throttle_vm_writeout(), which might delay us until the combined total of
NR_UNSTABLE_NFS + NR_WRITEBACK falls below the dirty limit.

However unstable pages don't go away automagickally, they need a push. While
balance_dirty_pages() does this push, throttle_vm_writeout() doesn't. So we can
sit here ad infintum.

Hence I propose to remove the NR_UNSTABLE_NFS count from throttle_vm_writeout().

Akpm's recent GFP checks don't much change this picture, any __GFP_IO|__GFP_FS
alloc can still get stalled by this. It turns into a deadlock when swapping
over NFS.

Signed-off-by: Peter Zijlstra <[email-masked]>
---
 mm/page-writeback.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

Index: linux-2.6-git/mm/page-writeback.c
===================================================================
--- linux-2.6-git.orig/mm/page-writeback.c	2007-03-06 17:44:23.000000000 +0100
+++ linux-2.6-git/mm/page-writeback.c	2007-03-15 15:09:16.000000000 +0100
@@ -320,8 +320,7 @@ void throttle_vm_writeout(gfp_t gfp_mask
                  */
                 dirty_thresh += dirty_thresh / 10;      /* wheeee... */
 
-                if (global_page_state(NR_UNSTABLE_NFS) +
-			global_page_state(NR_WRITEBACK) <= dirty_thresh)
+                if (global_page_state(NR_WRITEBACK) <= dirty_thresh)
                         	break;
                 congestion_wait(WRITE, HZ/10);
         }

--