Re: [PATCH v3 1/3] mm: replace migrate_prep with lru_add_drain_all

[Michal Hocko <mhocko@suse.com>]

On Wed 10-03-21 08:14:27, Minchan Kim wrote:
> Currently, migrate_prep is merely a wrapper of lru_cache_add_all.
> There is not much to gain from having additional abstraction.
> 
> Use lru_add_drain_all instead of migrate_prep, which would be more
> descriptive.
> 
> note: migrate_prep_local in compaction.c changed into lru_add_drain
> to avoid CPU schedule cost with involving many other CPUs to keep
> keep old behavior.
> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>

Acked-by: Michal Hocko <mhocko@suse.com>

Btw. that migrate_prep_local likely needs revisiting. I really fail to
see why it is useful. It looks like just in case thing to me. If it is
needed then the comment should be describing why. Something for a
separate patch though.

> ---
>  include/linux/migrate.h |  5 -----
>  mm/compaction.c         |  3 ++-
>  mm/mempolicy.c          |  4 ++--
>  mm/migrate.c            | 24 +-----------------------
>  mm/page_alloc.c         |  2 +-
>  mm/swap.c               |  5 +++++
>  6 files changed, 11 insertions(+), 32 deletions(-)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 3a389633b68f..6155d97ec76c 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -45,8 +45,6 @@ extern struct page *alloc_migration_target(struct page *page, unsigned long priv
>  extern int isolate_movable_page(struct page *page, isolate_mode_t mode);
>  extern void putback_movable_page(struct page *page);
>  
> -extern void migrate_prep(void);
> -extern void migrate_prep_local(void);
>  extern void migrate_page_states(struct page *newpage, struct page *page);
>  extern void migrate_page_copy(struct page *newpage, struct page *page);
>  extern int migrate_huge_page_move_mapping(struct address_space *mapping,
> @@ -66,9 +64,6 @@ static inline struct page *alloc_migration_target(struct page *page,
>  static inline int isolate_movable_page(struct page *page, isolate_mode_t mode)
>  	{ return -EBUSY; }
>  
> -static inline int migrate_prep(void) { return -ENOSYS; }
> -static inline int migrate_prep_local(void) { return -ENOSYS; }
> -
>  static inline void migrate_page_states(struct page *newpage, struct page *page)
>  {
>  }
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e04f4476e68e..3be017ececc0 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2319,7 +2319,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>  	trace_mm_compaction_begin(start_pfn, cc->migrate_pfn,
>  				cc->free_pfn, end_pfn, sync);
>  
> -	migrate_prep_local();
> +	/* lru_add_drain_all could be expensive with involving other CPUs */
> +	lru_add_drain();
>  
>  	while ((ret = compact_finished(cc)) == COMPACT_CONTINUE) {
>  		int err;
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index ab51132547b8..fc024e97be37 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1124,7 +1124,7 @@ int do_migrate_pages(struct mm_struct *mm, const nodemask_t *from,
>  	int err = 0;
>  	nodemask_t tmp;
>  
> -	migrate_prep();
> +	lru_add_drain_all();
>  
>  	mmap_read_lock(mm);
>  
> @@ -1323,7 +1323,7 @@ static long do_mbind(unsigned long start, unsigned long len,
>  
>  	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
>  
> -		migrate_prep();
> +		lru_add_drain_all();
>  	}
>  	{
>  		NODEMASK_SCRATCH(scratch);
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 62b81d5257aa..45f925e10f5a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -57,28 +57,6 @@
>  
>  #include "internal.h"
>  
> -/*
> - * migrate_prep() needs to be called before we start compiling a list of pages
> - * to be migrated using isolate_lru_page(). If scheduling work on other CPUs is
> - * undesirable, use migrate_prep_local()
> - */
> -void migrate_prep(void)
> -{
> -	/*
> -	 * Clear the LRU lists so pages can be isolated.
> -	 * Note that pages may be moved off the LRU after we have
> -	 * drained them. Those pages will fail to migrate like other
> -	 * pages that may be busy.
> -	 */
> -	lru_add_drain_all();
> -}
> -
> -/* Do the necessary work of migrate_prep but not if it involves other CPUs */
> -void migrate_prep_local(void)
> -{
> -	lru_add_drain();
> -}
> -
>  int isolate_movable_page(struct page *page, isolate_mode_t mode)
>  {
>  	struct address_space *mapping;
> @@ -1769,7 +1747,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>  	int start, i;
>  	int err = 0, err1;
>  
> -	migrate_prep();
> +	lru_add_drain_all();
>  
>  	for (i = start = 0; i < nr_pages; i++) {
>  		const void __user *p;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2e8348936df8..f05a8db741ca 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8467,7 +8467,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>  		.gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL,
>  	};
>  
> -	migrate_prep();
> +	lru_add_drain_all();
>  
>  	while (pfn < end || !list_empty(&cc->migratepages)) {
>  		if (fatal_signal_pending(current)) {
> diff --git a/mm/swap.c b/mm/swap.c
> index 31b844d4ed94..441d1ae1f285 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -729,6 +729,11 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
>  }
>  
>  /*
> + * lru_add_drain_all() usually needs to be called before we start compiling
> + * a list of pages to be migrated using isolate_lru_page(). Note that pages
> + * may be moved off the LRU after we have drained them. Those pages will
> + * fail to migrate like other pages that may be busy.
> + *
>   * Doesn't need any cpu hotplug locking because we do rely on per-cpu
>   * kworkers being shut down before our page_alloc_cpu_dead callback is
>   * executed on the offlined cpu.
> -- 
> 2.30.1.766.gb4fecdf3b7-goog

-- 
Michal Hocko
SUSE Labs