[PATCH] mm: don't discard unused swap slots by default

[Christoph Hellwig <hch@lst.de>]

Current TRIM/UNMAP/etc implementation are slow enough that discarding
small chunk during run time is a bad idea.  So only discard the whole
swap space on swapon by default, but require the admin to enable it
for run-time discards using the new vm.discard_swapspace sysctl.


Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: linux-2.6/include/linux/swap.h
===================================================================
--- linux-2.6.orig/include/linux/swap.h	2009-11-27 11:50:47.319003920 +0100
+++ linux-2.6/include/linux/swap.h	2009-11-27 11:51:55.617286868 +0100
@@ -247,6 +247,7 @@ extern unsigned long mem_cgroup_shrink_n
 extern int __isolate_lru_page(struct page *page, int mode, int file);
 extern unsigned long shrink_all_memory(unsigned long nr_pages);
 extern int vm_swappiness;
+extern int vm_discard_swapspace;
 extern int remove_mapping(struct address_space *mapping, struct page *page);
 extern long vm_total_pages;
 
Index: linux-2.6/kernel/sysctl.c
===================================================================
--- linux-2.6.orig/kernel/sysctl.c	2009-11-27 11:49:02.935254088 +0100
+++ linux-2.6/kernel/sysctl.c	2009-11-27 11:53:10.333006621 +0100
@@ -1163,6 +1163,16 @@ static struct ctl_table vm_table[] = {
 		.extra1		= &zero,
 		.extra2		= &one_hundred,
 	},
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "discard_swapspace",
+		.data		= &vm_discard_swapspace,
+		.maxlen		= sizeof(vm_discard_swapspace),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
 #ifdef CONFIG_HUGETLB_PAGE
 	 {
 		.procname	= "nr_hugepages",
Index: linux-2.6/mm/swapfile.c
===================================================================
--- linux-2.6.orig/mm/swapfile.c	2009-11-27 11:53:19.449254088 +0100
+++ linux-2.6/mm/swapfile.c	2009-11-27 11:54:07.883255931 +0100
@@ -41,6 +41,7 @@ long nr_swap_pages;
 long total_swap_pages;
 static int swap_overflow;
 static int least_priority;
+int vm_discard_swapspace;
 
 static const char Bad_file[] = "Bad swap file entry ";
 static const char Unused_file[] = "Unused swap file entry ";
@@ -1978,7 +1979,7 @@ SYSCALL_DEFINE2(swapon, const char __use
 			p->flags |= SWP_SOLIDSTATE;
 			p->cluster_next = 1 + (random32() % p->highest_bit);
 		}
-		if (discard_swap(p) == 0)
+		if (discard_swap(p) == 0 && vm_discard_swapspace)
 			p->flags |= SWP_DISCARDABLE;
 	}
 

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