linux-kernel - Re: [PATCH v3 1/3] mm/compaction: enable compacting >0 order folios.

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Date: Fri, 09 Feb 2024 15:44:35 -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 15:43, Vlastimil Babka wrote:

> On 2/9/24 20:25, Zi Yan wrote:
>> 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) {
>
> Yeah but I'd handle the !cc->alloc_contig first as that ends with a goto,
> and then the rest doesn't need to be "} else { ... }" with extra identation

OK. No problem.


--
Best Regards,
Yan, Zi

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