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

[Joonsoo Kim]

2015-12-14 19:07 GMT+09:00 Vlastimil Babka <vbabka@xxxxxxx>:
> 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...

Okay. Will send another patch for this clean-up on next spin.

Thanks.

>
>> ---
>>   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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