Re: [PATCH v5 2/3] mm/compaction: add support for >0 order folio memory compaction.

[Zi Yan <ziy@nvidia.com>]

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On 15 Feb 2024, at 11:07, Vlastimil Babka wrote:

> On 2/14/24 23:04, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> Before last commit, memory compaction only migrates order-0 folios and
>> skips >0 order folios.  Last commit splits all >0 order folios during
>> compaction.  This commit migrates >0 order folios during compaction by
>> keeping isolated free pages at their original size without splitting them
>> into order-0 pages and using them directly during migration process.
>>
>> What is different from the prior implementation:
>> 1. All isolated free pages are kept in a NR_PAGE_ORDERS array of page
>>    lists, where each page list stores free pages in the same order.
>> 2. All free pages are not post_alloc_hook() processed nor buddy pages,
>>    although their orders are stored in first page's private like buddy
>>    pages.
>> 3. During migration, in new page allocation time (i.e., in
>>    compaction_alloc()), free pages are then processed by post_alloc_hook().
>>    When migration fails and a new page is returned (i.e., in
>>    compaction_free()), free pages are restored by reversing the
>>    post_alloc_hook() operations using newly added
>>    free_pages_prepare_fpi_none().
>>
>> Step 3 is done for a latter optimization that splitting and/or merging
>> free pages during compaction becomes easier.
>>
>> Note: without splitting free pages, compaction can end prematurely due to
>> migration will return -ENOMEM even if there is free pages.  This happens
>> when no order-0 free page exist and compaction_alloc() return NULL.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Tested-by: Yu Zhao <yuzhao@google.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Thanks.

>
> Noticed a possible simplification:
>
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -447,6 +447,8 @@ extern void prep_compound_page(struct page *page, unsigned int order);
>>
>>  extern void post_alloc_hook(struct page *page, unsigned int order,
>>  					gfp_t gfp_flags);
>> +extern bool free_pages_prepare_fpi_none(struct page *page, unsigned int order);
>> +
>>  extern int user_min_free_kbytes;
>>
>>  extern void free_unref_page(struct page *page, unsigned int order);
>> @@ -481,7 +483,7 @@ int split_free_page(struct page *free_page,
>>   * completes when free_pfn <= migrate_pfn
>>   */
>>  struct compact_control {
>> -	struct list_head freepages;	/* List of free pages to migrate to */
>> +	struct list_head freepages[NR_PAGE_ORDERS];	/* List of free pages to migrate to */
>>  	struct list_head migratepages;	/* List of pages being migrated */
>>  	unsigned int nr_freepages;	/* Number of isolated free pages */
>>  	unsigned int nr_migratepages;	/* Number of pages to migrate */
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 7ae4b74c9e5c..e6e2ac722a82 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1179,6 +1179,12 @@ static __always_inline bool free_pages_prepare(struct page *page,
>>  	return true;
>>  }
>>
>> +__always_inline bool free_pages_prepare_fpi_none(struct page *page,
>> +			unsigned int order)
>> +{
>> +	return free_pages_prepare(page, order, FPI_NONE);
>
> Seems like free_pages_prepare() currently only passes  fpi_flags to
> should_skip_kasan_poison() and that ignores them. You could remove the
> parameter from both and declare and use free_pages_prepare(page, order)
> directly.

Got it. I can send a cleanup patch after this series. No, to avoid unnecessary
code churn, it is better to put a cleanup patch before this series and use
free_pages_prepare(). Will do it in v6.

>> +}
>> +
>>  /*
>>   * Frees a number of pages from the PCP lists
>>   * Assumes all pages on list are in same zone.


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Best Regards,
Yan, Zi

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