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@xxxxxx>
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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