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Date: Tue, 20 Feb 2024 10:27:33 -0500
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
"\"Huang, Ying\"" <ying.huang@...el.com>,
Ryan Roberts <ryan.roberts@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
"\"Matthew Wilcox (Oracle)\"" <willy@...radead.org>,
"\"Yin, Fengwei\"" <fengwei.yin@...el.com>, Yu Zhao <yuzhao@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>,
"\"Kirill A . Shutemov\"" <kirill.shutemov@...ux.intel.com>,
Johannes Weiner <hannes@...xchg.org>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
Kemeng Shi <shikemeng@...weicloud.com>,
Mel Gorman <mgorman@...hsingularity.net>,
Rohan Puri <rohan.puri15@...il.com>, Mcgrof Chamberlain <mcgrof@...nel.org>,
Adam Manzanares <a.manzanares@...sung.com>,
"\"Vishal Moola (Oracle)\"" <vishal.moola@...il.com>
Subject: Re: [PATCH v6 2/4] mm/compaction: enable compacting >0 order folios.
On 20 Feb 2024, at 4:03, David Hildenbrand wrote:
> On 16.02.24 18:04, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.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.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> Suggested-by: Huang Ying <ying.huang@...el.com>
>> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>> Tested-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Tested-by: Yu Zhao <yuzhao@...gle.com>
>> Cc: Adam Manzanares <a.manzanares@...sung.com>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: Kemeng Shi <shikemeng@...weicloud.com>
>> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>> Cc: Luis Chamberlain <mcgrof@...nel.org>
>> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: Vishal Moola (Oracle) <vishal.moola@...il.com>
>> Cc: Vlastimil Babka <vbabka@...e.cz>
>> Cc: Yin Fengwei <fengwei.yin@...el.com>
>> ---
>> mm/compaction.c | 66 ++++++++++++++++++++++++++++++++++++++-----------
>> 1 file changed, 52 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index cc801ce099b4..aa6aad805c4d 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;
>> }
>> +/*
>
>
> Can't you add these comments to the respective checks? Like
>
> static bool skip_isolation_on_order(int order, int target_order)
> {
> /*
> * Unless we are performing global compaction (targert_order <
> * 0), skip any folios that are larger than the target order: we
> * wouldn't be here if we'd have a free folio with the desired
> * target_order, so migrating this folio would likely fail
> * later.
> */
> if (target_order != -1 && order >= target_order)
> return true;
> /*
> * We limit memory compaction to pageblocks and won't try
> * creating free blocks of memory that are larger than that.
> */
> return order >= pageblock_order;
> }
>
> Then, add a simple expressive function documentation (if really required) that doesn't contain all these details.
>
OK. No problem.
>> + * 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,
>
> I'm pretty sure you meant "reduce" ?
Yes.
>
>> + * 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
>> @@ -947,7 +962,22 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>> valid_page = page;
>> }
>> - if (PageHuge(page) && cc->alloc_contig) {
>> + if (PageHuge(page)) {
>> + /*
>> + * skip hugetlbfs if we are not compacting for pages
>> + * bigger than its order. THPs and other compound pages
>> + * are handled below.
>> + */
>> + if (!cc->alloc_contig) {
>> + const unsigned int order = compound_order(page);
>> +
>> + if (order <= MAX_PAGE_ORDER) {
>> + low_pfn += (1UL << order) - 1;
>> + nr_scanned += (1UL << order) - 1;
>> + }
>> + goto isolate_fail;
>> + }
>> + /* for alloc_contig case */
>> if (locked) {
>> unlock_page_lruvec_irqrestore(locked, flags);
>> locked = NULL;
>> @@ -1008,21 +1038,24 @@ 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).
>> - * 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.
>> + * Regardless of being on LRU, compound pages such as THP
>> + * (hugetlbfs is handled above) are not to be compacted unless
>> + * we are attempting 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.
>> */
>> if (PageCompound(page) && !cc->alloc_contig) {
>> const unsigned int order = compound_order(page);
>> - if (likely(order <= MAX_PAGE_ORDER)) {
>> - low_pfn += (1UL << order) - 1;
>> - nr_scanned += (1UL << order) - 1;
>> + /* Skip based on page order and compaction target order. */
>> + if (skip_isolation_on_order(order, cc->order)) {
>> + if (order <= MAX_PAGE_ORDER) {
>> + low_pfn += (1UL << order) - 1;
>> + nr_scanned += (1UL << order) - 1;
>> + }
>> + goto isolate_fail;
>> }
>> - goto isolate_fail;
>> }
>> /*
>> @@ -1165,10 +1198,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>> }
>> /*
>> - * folio become large since the non-locked check,
>> - * and it's on LRU.
>> + * Check LRU folio order under the lock
>> */
>> - if (unlikely(folio_test_large(folio) && !cc->alloc_contig)) {
>> + 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);
>> @@ -1788,6 +1822,10 @@ 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_test_large(src) > 0)
>> + return NULL;
>
> Why the "> 0 " check ? Either it's large or it isn't.
Will fix it.
> Apart from that LGTM, but I am no compaction expert.
Thanks.
--
Best Regards,
Yan, Zi
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