mm: do not boost watermarks to avoid fragmentation for the DISCONTIG memory model
Mikulas Patocka reported that commit 1c30844d2dfe ("mm: reclaim small
amounts of memory when an external fragmentation event occurs") "broke"
memory management on parisc.
The machine is not NUMA but the DISCONTIG model creates three pgdats
even though it's a UMA machine for the following ranges
0) Start 0x0000000000000000 End 0x000000003fffffff Size 1024 MB
1) Start 0x0000000100000000 End 0x00000001bfdfffff Size 3070 MB
2) Start 0x0000004040000000 End 0x00000040ffffffff Size 3072 MB
Mikulas reported:
With the patch 1c30844d2, the kernel will incorrectly reclaim the
first zone when it fills up, ignoring the fact that there are two
completely free zones. Basiscally, it limits cache size to 1GiB.
For example, if I run:
# dd if=/dev/sda of=/dev/null bs=1M count=2048
- with the proper kernel, there should be "Buffers - 2GiB"
when this command finishes. With the patch 1c30844d2, buffers
will consume just 1GiB or slightly more, because the kernel was
incorrectly reclaiming them.
The page allocator and reclaim makes assumptions that pgdats really
represent NUMA nodes and zones represent ranges and makes decisions on
that basis. Watermark boosting for small pgdats leads to unexpected
results even though this would have behaved reasonably on SPARSEMEM.
DISCONTIG is essentially deprecated and even parisc plans to move to
SPARSEMEM so there is no need to be fancy, this patch simply disables
watermark boosting by default on DISCONTIGMEM.
Link: http://lkml.kernel.org/r/20190419094335.GJ18914@techsingularity.net
Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c6ce20a..6c6d9f1 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -266,7 +266,20 @@ compound_page_dtor * const compound_page_dtors[] = {
int min_free_kbytes = 1024;
int user_min_free_kbytes = -1;
+#ifdef CONFIG_DISCONTIGMEM
+/*
+ * DiscontigMem defines memory ranges as separate pg_data_t even if the ranges
+ * are not on separate NUMA nodes. Functionally this works but with
+ * watermark_boost_factor, it can reclaim prematurely as the ranges can be
+ * quite small. By default, do not boost watermarks on discontigmem as in
+ * many cases very high-order allocations like THP are likely to be
+ * unsupported and the premature reclaim offsets the advantage of long-term
+ * fragmentation avoidance.
+ */
+int watermark_boost_factor __read_mostly;
+#else
int watermark_boost_factor __read_mostly = 15000;
+#endif
int watermark_scale_factor = 10;
static unsigned long nr_kernel_pages __initdata;