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Subject[PATCH][1/2] adjust dirty threshold for lowmem-only mappings
Simply running "dd if=/dev/zero of=/dev/hd<one you can miss>" will
result in OOM kills, with the dirty pagecache completely filling up
lowmem. This patch is part 1 to fixing that problem.

This patch effectively lowers the dirty limit for mappings which cannot
be cached in highmem, counting the dirty limit as a percentage of lowmem
instead. This should prevent heavy block device writers from pushing
the VM over the edge and triggering OOM kills.

Signed-off-by: Rik van Riel <riel@redhat.com>


--- linux-2.6.9/mm/page-writeback.c.highmem 2004-12-16 11:22:48.193641312
-0500
+++ linux-2.6.9/mm/page-writeback.c 2004-12-16 11:30:00.565676290 -0500
@@ -133,18 +133,28 @@
* clamping level.
*/
static void
-get_dirty_limits(struct writeback_state *wbs, long *pbackground, long *pdirty)
+get_dirty_limits(struct writeback_state *wbs, long *pbackground, long *pdirty,
struct address_space *mapping)
{
int background_ratio; /* Percentages */
int dirty_ratio;
int unmapped_ratio;
long background;
long dirty;
+ unsigned long available_memory = total_pages;
struct task_struct *tsk;

get_writeback_state(wbs);

- unmapped_ratio = 100 - (wbs->nr_mapped * 100) / total_pages;
+#ifdef CONFIG_HIGHMEM
+ /*
+ * If this mapping can only allocate from low memory,
+ * we exclude high memory from our count.
+ */
+ if (mapping && !(mapping_gfp_mask(mapping) & __GFP_HIGHMEM))
+ available_memory -= totalhigh_pages;
+#endif
+
+ unmapped_ratio = 100 - (wbs->nr_mapped * 100) / available_memory;

dirty_ratio = vm_dirty_ratio;
if (dirty_ratio > unmapped_ratio / 2)
@@ -194,7 +204,8 @@
.nr_to_write = write_chunk,
};

- get_dirty_limits(&wbs, &background_thresh, &dirty_thresh);
+ get_dirty_limits(&wbs, &background_thresh,
+ &dirty_thresh, mapping);
nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable;
if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
break;
@@ -210,7 +221,7 @@
if (nr_reclaimable) {
writeback_inodes(&wbc);
get_dirty_limits(&wbs, &background_thresh,
- &dirty_thresh);
+ &dirty_thresh, mapping);
nr_reclaimable = wbs.nr_dirty + wbs.nr_unstable;
if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
break;
@@ -283,7 +294,7 @@
long dirty_thresh;

for ( ; ; ) {
- get_dirty_limits(&wbs, &background_thresh, &dirty_thresh);
+ get_dirty_limits(&wbs, &background_thresh, &dirty_thresh,
NULL);

/*
* Boost the allowable dirty threshold a bit for page
@@ -318,7 +329,7 @@
long background_thresh;
long dirty_thresh;

- get_dirty_limits(&wbs, &background_thresh, &dirty_thresh);
+ get_dirty_limits(&wbs, &background_thresh, &dirty_thresh,
NULL);
if (wbs.nr_dirty + wbs.nr_unstable < background_thresh
&& min_pages <= 0)
break;
-
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