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Date: Fri, 09 Feb 2024 14:25:00 -0500
From: Zi Yan <ziy@...dia.com>
To: Vlastimil Babka <vbabka@...e.cz>
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>,
David Hildenbrand <david@...hat.com>,
"\"Yin, Fengwei\"" <fengwei.yin@...el.com>, Yu Zhao <yuzhao@...gle.com>,
"\"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 v3 1/3] mm/compaction: enable compacting >0 order folios.
On 9 Feb 2024, at 9:32, Vlastimil Babka wrote:
> On 2/2/24 17:15, 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.
>>
>> Suggested-by: Huang Ying <ying.huang@...el.com>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>> mm/compaction.c | 43 +++++++++++++++++++++++++++++++++++--------
>> 1 file changed, 35 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 4add68d40e8d..e43e898d2c77 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
>> @@ -1010,7 +1025,7 @@ 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.
>> @@ -1018,11 +1033,18 @@ 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_PAGE_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)) {
>
> Hm I'd try to avoid a new PageHuge() test here.
>
> Earlier we have a block that does
> if (PageHuge(page) && cc->alloc_contig) {
> ...
>
> think I'd rather rewrite it to handle the PageHuge() case completely and
> just make it skip the 1UL << order pages there for !cc->alloc_config. Even
> if it means duplicating a bit of the low_pfn and nr_scanned bumping code.
>
> Which reminds me the PageHuge() check there is probably still broken ATM:
>
> https://lore.kernel.org/all/8fa1c95c-4749-33dd-42ba-243e492ab109@suse.cz/
>
> Even better reason not to add another one.
> If the huge page materialized since the first check, we should bail out when
> testing PageLRU later anyway.
OK, so basically something like:
if (PageHuge(page)) {
if (cc->alloc_contig) {
// existing code for PageHuge(page) && cc->allc_contig
} else {
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;
}
}
...
if (PageCompound(page) && !cc->alloc_contig) {
const unsigned int order = compound_order(page);
/* 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;
}
}
>
>> + if (order <= MAX_PAGE_ORDER) {
>> + low_pfn += (1UL << order) - 1;
>> + nr_scanned += (1UL << order) - 1;
>> + }
>> + goto isolate_fail;
>> }
>> - goto isolate_fail;
>> }
>>
>> /*
>> @@ -1165,10 +1187,11 @@ 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);
>> @@ -1786,6 +1809,10 @@ 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;
>> +
>> if (list_empty(&cc->freepages)) {
>> isolate_freepages(cc);
>>
--
Best Regards,
Yan, Zi
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