Re: [PATCH v2] mm/migrate: put dest folio on deferred split list if source was there.

[Zi Yan]

On 12 Mar 2024, at 3:27, Baolin Wang wrote:

> On 2024/3/12 03:58, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> Commit 616b8371539a6 ("mm: thp: enable thp migration in generic path")
>> did not check if a THP is on deferred split list before migration, thus,
>> the destination THP is never put on deferred split list even if the source
>> THP might be. The opportunity of reclaiming free pages in a partially
>> mapped THP during deferred list scanning is lost, but no other harmful
>> consequence is present[1]. Checking source folio deferred split list
>> status before page unmapped and add destination folio to the list if
>> source was after migration.
>>
>> [1]: https://lore.kernel.org/linux-mm/03CE3A00-917C-48CC-8E1C-6A98713C817C@xxxxxxxxxx/
>>
>>  From v1:
>> 1. Used dst to get correct deferred split list after migration
>>     (per Ryan Roberts).
>>
>> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> ---
>>   mm/huge_memory.c | 22 ----------------------
>>   mm/internal.h    | 23 +++++++++++++++++++++++
>>   mm/migrate.c     | 26 +++++++++++++++++++++++++-
>>   3 files changed, 48 insertions(+), 23 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 9859aa4f7553..c6d4d0cdf4b3 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -766,28 +766,6 @@ pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
>>   	return pmd;
>>   }
>>  -#ifdef CONFIG_MEMCG
>> -static inline
>> -struct deferred_split *get_deferred_split_queue(struct folio *folio)
>> -{
>> -	struct mem_cgroup *memcg = folio_memcg(folio);
>> -	struct pglist_data *pgdat = NODE_DATA(folio_nid(folio));
>> -
>> -	if (memcg)
>> -		return &memcg->deferred_split_queue;
>> -	else
>> -		return &pgdat->deferred_split_queue;
>> -}
>> -#else
>> -static inline
>> -struct deferred_split *get_deferred_split_queue(struct folio *folio)
>> -{
>> -	struct pglist_data *pgdat = NODE_DATA(folio_nid(folio));
>> -
>> -	return &pgdat->deferred_split_queue;
>> -}
>> -#endif
>> -
>>   void folio_prep_large_rmappable(struct folio *folio)
>>   {
>>   	if (!folio || !folio_test_large(folio))
>> diff --git a/mm/internal.h b/mm/internal.h
>> index d1c69119b24f..8fa36e84463a 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -1107,6 +1107,29 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>>   				   unsigned long addr, pmd_t *pmd,
>>   				   unsigned int flags);
>>  +#ifdef CONFIG_MEMCG
>> +static inline
>> +struct deferred_split *get_deferred_split_queue(struct folio *folio)
>> +{
>> +	struct mem_cgroup *memcg = folio_memcg(folio);
>> +	struct pglist_data *pgdat = NODE_DATA(folio_nid(folio));
>> +
>> +	if (memcg)
>> +		return &memcg->deferred_split_queue;
>> +	else
>> +		return &pgdat->deferred_split_queue;
>> +}
>> +#else
>> +static inline
>> +struct deferred_split *get_deferred_split_queue(struct folio *folio)
>> +{
>> +	struct pglist_data *pgdat = NODE_DATA(folio_nid(folio));
>> +
>> +	return &pgdat->deferred_split_queue;
>> +}
>> +#endif
>> +
>> +
>>   /*
>>    * mm/mmap.c
>>    */
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 73a052a382f1..591e65658535 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -20,6 +20,7 @@
>>   #include <linux/pagemap.h>
>>   #include <linux/buffer_head.h>
>>   #include <linux/mm_inline.h>
>> +#include <linux/mmzone.h>
>>   #include <linux/nsproxy.h>
>>   #include <linux/ksm.h>
>>   #include <linux/rmap.h>
>> @@ -1037,7 +1038,10 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>>   enum {
>>   	PAGE_WAS_MAPPED = BIT(0),
>>   	PAGE_WAS_MLOCKED = BIT(1),
>> -	PAGE_OLD_STATES = PAGE_WAS_MAPPED | PAGE_WAS_MLOCKED,
>> +	PAGE_WAS_ON_DEFERRED_LIST = BIT(2),
>> +	PAGE_OLD_STATES = PAGE_WAS_MAPPED |
>> +			  PAGE_WAS_MLOCKED |
>> +			  PAGE_WAS_ON_DEFERRED_LIST,
>>   };
>>    static void __migrate_folio_record(struct folio *dst,
>> @@ -1168,6 +1172,17 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>>   		folio_lock(src);
>>   	}
>>   	locked = true;
>> +	if (folio_test_large_rmappable(src) &&
>
> IMO, you should check folio_test_large() before calling folio_test_large_rmappable(), since the PG_large_rmappable flag is stored in the first tail page.

You are right. Ryan also pointed this out in another email. Will fix. Thanks.


--
Best Regards,
Yan, Zi

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