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

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Message-ID: <7DE48248-2210-487D-82A0-5FE9F2C14871@nvidia.com>
Date: Tue, 19 Mar 2024 21:30:00 -0400
From: Zi Yan <ziy@...dia.com>
To: "\"Matthew Wilcox (Oracle)\"" <willy@...radead.org>
Cc: linux-mm@...ck.org, SeongJae Park <sj@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "\"Matthew Wilcox (Oracle)\"" <willy@...radead.org>,
 Yang Shi <shy828301@...il.com>, Huang Ying <ying.huang@...el.com>,
 "\"Kirill A . Shutemov\"" <kirill.shutemov@...ux.intel.com>,
 Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "\"Yin, Fengwei\"" <fengwei.yin@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/migrate: split source folio if it is on deferred
 split list

Hi Matthew,

>>>> 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.
>
> Actually, since get_deferred_split_queue() is used in mm/migrate.c, that
> part needs to be guarded by CONFIG_TRANSPARENT_HUGEPAGE as well.

_deferred_list is not hidden behind CONFIG_TRANSPARENT_HUGEPAGE but
its users are all behind CONFIG_TRANSPARENT_HUGEPAGE. Should it be moved
as well? Or the other way around, moving _deferred_list users out?

Large folios should work without THP, since they would be like THP without
PMD mappings, namely mTHP. But mTHP is considered a subset of THP. It seems
confusing. Maybe hiding _deferred_list behind CONFIG_TRANSPARENT_HUGEPAGE
is better, or just do not change.

--
Best Regards,
Yan, Zi

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