Re: [PATCH 1/2] mm/compaction: fix invalid free_pfn and compact_cached_free_pfn

From: Vlastimil Babka
Date: Mon Dec 14 2015 - 05:07:14 EST


On 12/14/2015 06:02 AM, Joonsoo Kim wrote:
free_pfn and compact_cached_free_pfn are the pointer that remember
restart position of freepage scanner. When they are reset or invalid,
we set them to zone_end_pfn because freepage scanner works in reverse
direction. But, because zone range is defined as [zone_start_pfn,
zone_end_pfn), zone_end_pfn is invalid to access. Therefore, we should
not store it to free_pfn and compact_cached_free_pfn. Instead, we need
to store zone_end_pfn - 1 to them. There is one more thing we should
consider. Freepage scanner scan reversely by pageblock unit. If free_pfn
and compact_cached_free_pfn are set to middle of pageblock, it regards
that sitiation as that it already scans front part of pageblock so we
lose opportunity to scan there. To fix-up, this patch do round_down()
to guarantee that reset position will be pageblock aligned.

Note that thanks to the current pageblock_pfn_to_page() implementation,
actual access to zone_end_pfn doesn't happen until now. But, following
patch will change pageblock_pfn_to_page() so this patch is needed
from now on.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Note that until now in compaction we've used basically an open-coded round_down(), and ALIGN() for rounding up. You introduce a first use of round_down(), and it would be nice to standardize on round_down() and round_up() everywhere. I think it's more obvious than open-coding and ALIGN() (which doesn't tell the reader if it's aligning up or down). Hopefully they really do the same thing and there are no caveats...

---
mm/compaction.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 585de54..56fa321 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -200,7 +200,8 @@ static void reset_cached_positions(struct zone *zone)
{
zone->compact_cached_migrate_pfn[0] = zone->zone_start_pfn;
zone->compact_cached_migrate_pfn[1] = zone->zone_start_pfn;
- zone->compact_cached_free_pfn = zone_end_pfn(zone);
+ zone->compact_cached_free_pfn =
+ round_down(zone_end_pfn(zone) - 1, pageblock_nr_pages);
}

/*
@@ -1371,11 +1372,11 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
*/
cc->migrate_pfn = zone->compact_cached_migrate_pfn[sync];
cc->free_pfn = zone->compact_cached_free_pfn;
- if (cc->free_pfn < start_pfn || cc->free_pfn > end_pfn) {
- cc->free_pfn = end_pfn & ~(pageblock_nr_pages-1);
+ if (cc->free_pfn < start_pfn || cc->free_pfn >= end_pfn) {
+ cc->free_pfn = round_down(end_pfn - 1, pageblock_nr_pages);
zone->compact_cached_free_pfn = cc->free_pfn;
}
- if (cc->migrate_pfn < start_pfn || cc->migrate_pfn > end_pfn) {
+ if (cc->migrate_pfn < start_pfn || cc->migrate_pfn >= end_pfn) {
cc->migrate_pfn = start_pfn;
zone->compact_cached_migrate_pfn[0] = cc->migrate_pfn;
zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;


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