[v1,1/4] mm/compaction: enable compacting >0 order folios.
Commit Message
From: Zi Yan <ziy@nvidia.com>
migrate_pages() supports >0 order folio migration and during compaction,
even if compaction_alloc() cannot provide >0 order free pages,
migrate_pages() can split the source page and try to migrate the base pages
from the split. It can be a baseline and start point for adding support for
compacting >0 order folios.
Suggested-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
mm/compaction.c | 57 ++++++++++++++++++++++++++++++++++++-------------
1 file changed, 42 insertions(+), 15 deletions(-)
Comments
On Mon, Nov 13, 2023 at 12:01:54PM -0500, Zi Yan wrote:
> + /* this makes migrate_pages() split the source page and retry */
> + if (folio_order(src) > 0)
> + return NULL;
Nit: folio_test_large() is more efficient than folio_order() > 0.
The former simply tests the bit, while the second tests the bit, then
loads folio->_order to check it's >0. We know it will be, but there's
no way to tell gcc that if the bit is set, this value is definitely not 0.
On 13 Nov 2023, at 13:30, Matthew Wilcox wrote:
> On Mon, Nov 13, 2023 at 12:01:54PM -0500, Zi Yan wrote:
>> + /* this makes migrate_pages() split the source page and retry */
>> + if (folio_order(src) > 0)
>> + return NULL;
>
> Nit: folio_test_large() is more efficient than folio_order() > 0.
> The former simply tests the bit, while the second tests the bit, then
> loads folio->_order to check it's >0. We know it will be, but there's
> no way to tell gcc that if the bit is set, this value is definitely not 0.
Got it. Make sense. Will change it in next version. Thanks.
--
Best Regards,
Yan, Zi
On 11/14/2023 1:01 AM, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
>
> migrate_pages() supports >0 order folio migration and during compaction,
> even if compaction_alloc() cannot provide >0 order free pages,
> migrate_pages() can split the source page and try to migrate the base pages
> from the split. It can be a baseline and start point for adding support for
> compacting >0 order folios.
>
> Suggested-by: Huang Ying <ying.huang@intel.com>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
> mm/compaction.c | 57 ++++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 42 insertions(+), 15 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 01ba298739dd..5217dd35b493 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -816,6 +816,21 @@ static bool too_many_isolated(struct compact_control *cc)
> return too_many;
> }
>
> +/*
> + * 1. if the page order is larger than or equal to target_order (i.e.,
> + * cc->order and when it is not -1 for global compaction), skip it since
> + * target_order already indicates no free page with larger than target_order
> + * exists and later migrating it will most likely fail;
> + *
> + * 2. compacting > pageblock_order pages does not improve memory fragmentation,
> + * skip them;
> + */
> +static bool skip_isolation_on_order(int order, int target_order)
> +{
> + return (target_order != -1 && order >= target_order) ||
> + order >= pageblock_order;
> +}
> +
> /**
> * isolate_migratepages_block() - isolate all migrate-able pages within
> * a single pageblock
> @@ -1009,7 +1024,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> /*
> * Regardless of being on LRU, compound pages such as THP and
> * hugetlbfs are not to be compacted unless we are attempting
> - * an allocation much larger than the huge page size (eg CMA).
> + * an allocation larger than the compound page size.
> * We can potentially save a lot of iterations if we skip them
> * at once. The check is racy, but we can consider only valid
> * values and the only danger is skipping too much.
> @@ -1017,11 +1032,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> if (PageCompound(page) && !cc->alloc_contig) {
> const unsigned int order = compound_order(page);
>
> - if (likely(order <= MAX_ORDER)) {
> - low_pfn += (1UL << order) - 1;
> - nr_scanned += (1UL << order) - 1;
> + /*
> + * Skip based on page order and compaction target order
> + * and skip hugetlbfs pages.
> + */
> + if (skip_isolation_on_order(order, cc->order) ||
> + PageHuge(page)) {
> + if (order <= MAX_ORDER) {
> + low_pfn += (1UL << order) - 1;
> + nr_scanned += (1UL << order) - 1;
> + }
> + goto isolate_fail;
> }
> - goto isolate_fail;
> }
>
> /*
> @@ -1144,17 +1166,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> goto isolate_abort;
> }
> }
> + }
>
> - /*
> - * folio become large since the non-locked check,
> - * and it's on LRU.
> - */
> - if (unlikely(folio_test_large(folio) && !cc->alloc_contig)) {
> - low_pfn += folio_nr_pages(folio) - 1;
> - nr_scanned += folio_nr_pages(folio) - 1;
> - folio_set_lru(folio);
> - goto isolate_fail_put;
> - }
> + /*
> + * Check LRU folio order under the lock
> + */
> + if (unlikely(skip_isolation_on_order(folio_order(folio),
> + cc->order) &&
> + !cc->alloc_contig)) {
> + low_pfn += folio_nr_pages(folio) - 1;
> + nr_scanned += folio_nr_pages(folio) - 1;
> + folio_set_lru(folio);
> + goto isolate_fail_put;
> }
Why was this part moved out of the 'if (lruvec != locked)' block? If we
hold the lru lock, then we do not need to check again, right?
>> @@ -1144,17 +1166,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>> goto isolate_abort;
>> }
>> }
>> + }
>> - /*
>> - * folio become large since the non-locked check,
>> - * and it's on LRU.
>> - */
>> - if (unlikely(folio_test_large(folio) && !cc->alloc_contig)) {
>> - low_pfn += folio_nr_pages(folio) - 1;
>> - nr_scanned += folio_nr_pages(folio) - 1;
>> - folio_set_lru(folio);
>> - goto isolate_fail_put;
>> - }
>> + /*
>> + * Check LRU folio order under the lock
>> + */
>> + if (unlikely(skip_isolation_on_order(folio_order(folio),
>> + cc->order) &&
>> + !cc->alloc_contig)) {
>> + low_pfn += folio_nr_pages(folio) - 1;
>> + nr_scanned += folio_nr_pages(folio) - 1;
>> + folio_set_lru(folio);
>> + goto isolate_fail_put;
>> }
>
> Why was this part moved out of the 'if (lruvec != locked)' block? If we hold the lru lock, then we do not need to check again, right?
Probably I messed this up during rebase. Thank you for pointing this out.
Will fix it in the next version.
--
Best Regards,
Yan, Zi
@@ -816,6 +816,21 @@ static bool too_many_isolated(struct compact_control *cc)
return too_many;
}
+/*
+ * 1. if the page order is larger than or equal to target_order (i.e.,
+ * cc->order and when it is not -1 for global compaction), skip it since
+ * target_order already indicates no free page with larger than target_order
+ * exists and later migrating it will most likely fail;
+ *
+ * 2. compacting > pageblock_order pages does not improve memory fragmentation,
+ * skip them;
+ */
+static bool skip_isolation_on_order(int order, int target_order)
+{
+ return (target_order != -1 && order >= target_order) ||
+ order >= pageblock_order;
+}
+
/**
* isolate_migratepages_block() - isolate all migrate-able pages within
* a single pageblock
@@ -1009,7 +1024,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
/*
* Regardless of being on LRU, compound pages such as THP and
* hugetlbfs are not to be compacted unless we are attempting
- * an allocation much larger than the huge page size (eg CMA).
+ * an allocation larger than the compound page size.
* We can potentially save a lot of iterations if we skip them
* at once. The check is racy, but we can consider only valid
* values and the only danger is skipping too much.
@@ -1017,11 +1032,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
if (PageCompound(page) && !cc->alloc_contig) {
const unsigned int order = compound_order(page);
- if (likely(order <= MAX_ORDER)) {
- low_pfn += (1UL << order) - 1;
- nr_scanned += (1UL << order) - 1;
+ /*
+ * Skip based on page order and compaction target order
+ * and skip hugetlbfs pages.
+ */
+ if (skip_isolation_on_order(order, cc->order) ||
+ PageHuge(page)) {
+ if (order <= MAX_ORDER) {
+ low_pfn += (1UL << order) - 1;
+ nr_scanned += (1UL << order) - 1;
+ }
+ goto isolate_fail;
}
- goto isolate_fail;
}
/*
@@ -1144,17 +1166,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
goto isolate_abort;
}
}
+ }
- /*
- * folio become large since the non-locked check,
- * and it's on LRU.
- */
- if (unlikely(folio_test_large(folio) && !cc->alloc_contig)) {
- low_pfn += folio_nr_pages(folio) - 1;
- nr_scanned += folio_nr_pages(folio) - 1;
- folio_set_lru(folio);
- goto isolate_fail_put;
- }
+ /*
+ * Check LRU folio order under the lock
+ */
+ if (unlikely(skip_isolation_on_order(folio_order(folio),
+ cc->order) &&
+ !cc->alloc_contig)) {
+ low_pfn += folio_nr_pages(folio) - 1;
+ nr_scanned += folio_nr_pages(folio) - 1;
+ folio_set_lru(folio);
+ goto isolate_fail_put;
}
/* The folio is taken off the LRU */
@@ -1764,6 +1787,10 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
struct compact_control *cc = (struct compact_control *)data;
struct folio *dst;
+ /* this makes migrate_pages() split the source page and retry */
+ if (folio_order(src) > 0)
+ return NULL;
+
if (list_empty(&cc->freepages)) {
isolate_freepages(cc);