[PATCH] Deadlock during heavy write activity to userspace NFS server on local NFS mount

[Avi Kivity <avi@exanet.com>]

On heavy write activity, allocators wait synchronously for kswapd to
free some memory. But if kswapd is freeing memory via a userspace NFS
server, that server could be waiting for kswapd, and the system seizes
instantly.

This patch (against RHEL 2.4.21-15EL, but should apply either literally
or conceptually to other kernels) allows a process to declare itself as
kswapd's little helper, and thus will not have to wait on kswapd.

--- a/include/linux/prctl.h	2003-10-23 09:00:00.000000000 +0200
+++ b/include/linux/prctl.h	2004-07-21 13:43:01.000000000 +0300
@@ -43,5 +43,10 @@
  # define PR_TIMING_TIMESTAMP	1	/* Accurate timestamp based
  						   process timing */

+/* Get/set PF_MEMALLOC task flag bit */
+#define PR_GET_KSWAPD_HELPER 15
+#define PR_SET_KSWAPD_HELPER 16
+
+
  #endif /* _LINUX_PRCTL_H */
--- a/kernel/sys.c	2003-10-23 09:00:00.000000000 +0200
+++ b/kernel/sys.c	2004-07-21 13:42:59.000000000 +0300
@@ -1400,6 +1400,22 @@ asmlinkage long sys_prctl(int option, un
  			}
  			current->keep_capabilities = arg2;
  			break;
+		case PR_GET_KSWAPD_HELPER:
+			if (current->flags & PF_MEMALLOC)
+				error = 1;
+			break;
+		case PR_SET_KSWAPD_HELPER:
+			switch (arg2) {
+				case 0:
+					current->flags &= ~PF_MEMALLOC;
+					break;
+				case 1:
+					current->flags |= PF_MEMALLOC;
+					break;
+				default:
+					error = -EINVAL;
+			}
+			break;
  		default:
  			error = -EINVAL;
  			break;


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.