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

[SeongJae Park <sj@kernel.org>]

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,
SJ

[...]

-------------------------------------- >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 */


 /*