Re: [PATCH v3] mm/migrate: split source folio if it is on deferred split list

[Zi Yan <ziy@nvidia.com>]

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On 19 Mar 2024, at 21:08, SeongJae Park wrote:

> Hello,
>
> On Tue, 19 Mar 2024 11:47:53 -0400 Zi Yan <zi.yan@sent.com> wrote:
>
>> From: Zi Yan <ziy@nvidia.com>
>>
>> If the source folio is on deferred split list, it is likely some subpages
>> are not used. Split it before migration to avoid migrating unused subpages.
>>
>> 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].
>>
>> From v2:
>> 1. Split the source folio instead of migrating it (per Matthew Wilcox)[2].
>>
>> From v1:
>> 1. Used dst to get correct deferred split list after migration
>>    (per Ryan Roberts).
>>
>> [1]: https://lore.kernel.org/linux-mm/03CE3A00-917C-48CC-8E1C-6A98713C817C@nvidia.com/
>> [2]: https://lore.kernel.org/linux-mm/Ze_P6xagdTbcu1Kz@casper.infradead.org/
>>
>> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>  mm/huge_memory.c | 22 ------------------
>>  mm/internal.h    | 23 +++++++++++++++++++
>>  mm/migrate.c     | 60 +++++++++++++++++++++++++++++++++++++++---------
>>  3 files changed, 72 insertions(+), 33 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
>
> I found this breaks the build when CONFIG_TRANSPARENT_HUGEPAGE is not set, with
> below error:
>
>     .../lib/../mm/internal.h: In function 'get_deferred_split_queue':
>     .../lib/../mm/internal.h:1127:22: error: 'struct pglist_data' has no member named 'deferred_split_queue'
>      1127 |         return &pgdat->deferred_split_queue;
>           |                      ^~
>
> Since the code was in hugepage.c, maybe the above chunk need to be wrapped by
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE?  I confirmed below change is fixing the
> build on my setup.

Thanks. Will fix it in the next version.

>
> [...]
>
> -------------------------------------- >8 -------------------------------------
> diff --git a/mm/internal.h b/mm/internal.h
> index dce2b9f5e6cd..fe9f69ceb140 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1106,6 +1106,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>                                    unsigned long addr, pmd_t *pmd,
>                                    unsigned int flags);
>
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  #ifdef CONFIG_MEMCG
>  static inline
>  struct deferred_split *get_deferred_split_queue(struct folio *folio)
> @@ -1126,7 +1127,8 @@ struct deferred_split *get_deferred_split_queue(struct folio *folio)
>
>         return &pgdat->deferred_split_queue;
>  }
> -#endif
> +#endif /* CONFIG_MEMCG */
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>
>
>  /*


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Best Regards,
Yan, Zi

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