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

[SeongJae Park <sj@kernel.org>]

On Tue, 19 Mar 2024 21:16:37 -0400 Zi Yan <ziy@nvidia.com> wrote:

> [-- Attachment #1: Type: text/plain, Size: 4354 bytes --]
> 
> On 19 Mar 2024, at 21:09, Zi Yan wrote:
> 
> > 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.
> 
> Actually, since get_deferred_split_queue() is used in mm/migrate.c, that
> part needs to be guarded by CONFIG_TRANSPARENT_HUGEPAGE as well.

You're right.  I also just confirmed that build breaks with below error when
CONFIG_TRANSPARENT_HUGEPAGE is not set but CONFIG_MIGRATION is set.

    ERROR:root:.../mm/migrate.c: In function ‘migrate_pages_batch’:
    .../mm/migrate.c:1682:49: error: implicit declaration of function ‘get_deferred_split_queue’ [-Werror=implicit-function-declaration]
     1682 |                                                 get_deferred_split_queue(folio);
          |                                                 ^~~~~~~~~~~~~~~~~~~~~~~~


Thanks,
SJ

> 
> --
> Best Regards,
> Yan, Zi