Re: [PATCH 1/4] mm: migrate: move migration validation into numa_migrate_prep()

[Huang, Ying]

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

> Now there are 3 places will validate if a page can mirate or not, and
> some validations are performed later, which will waste some CPU to call
> numa_migrate_prep().
>
> Thus we can move all the migration validation into numa_migrate_prep(),
> which is more maintainable as well as saving some CPU resources. Another
> benefit is that it can serve as a preparation for supporting batch migration
> in do_numa_page() in future.
>
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---
>  mm/memory.c  | 19 +++++++++++++++++++
>  mm/migrate.c | 19 -------------------
>  2 files changed, 19 insertions(+), 19 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index d003076b218d..bee9b1e86ef0 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4747,6 +4747,25 @@ int numa_migrate_prep(struct page *page, struct vm_area_struct *vma,
>  		*flags |= TNF_FAULT_LOCAL;
>  	}
>  
> +	/*
> +	 * Don't migrate file pages that are mapped in multiple processes
> +	 * with execute permissions as they are probably shared libraries.
> +	 */
> +	if (page_mapcount(page) != 1 && page_is_file_lru(page) &&
> +	    (vma->vm_flags & VM_EXEC))
> +		return NUMA_NO_NODE;
> +
> +	/*
> +	 * Also do not migrate dirty pages as not all filesystems can move
> +	 * dirty pages in MIGRATE_ASYNC mode which is a waste of cycles.
> +	 */
> +	if (page_is_file_lru(page) && PageDirty(page))
> +		return NUMA_NO_NODE;
> +
> +	/* Do not migrate THP mapped by multiple processes */
> +	if (PageTransHuge(page) && total_mapcount(page) > 1)
> +		return NUMA_NO_NODE;
> +
>  	return mpol_misplaced(page, vma, addr);

In mpol_misplaced()->should_numa_migrate_memory(), accessing CPU and PID
will be recorded.  So the code change above will introduce some behavior
change.

How about move these checks into a separate function which is called
between numa_migrate_prep() and migrate_misplaced_page() after unlocking
PTL?

--
Best Regards,
Huang, Ying

>  }
>  
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e21d5a7e7447..9cc98fb1d6ec 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2485,10 +2485,6 @@ static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
>  
>  	VM_BUG_ON_PAGE(order && !PageTransHuge(page), page);
>  
> -	/* Do not migrate THP mapped by multiple processes */
> -	if (PageTransHuge(page) && total_mapcount(page) > 1)
> -		return 0;
> -
>  	/* Avoid migrating to a node that is nearly full */
>  	if (!migrate_balanced_pgdat(pgdat, nr_pages)) {
>  		int z;
> @@ -2533,21 +2529,6 @@ int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
>  	LIST_HEAD(migratepages);
>  	int nr_pages = thp_nr_pages(page);
>  
> -	/*
> -	 * Don't migrate file pages that are mapped in multiple processes
> -	 * with execute permissions as they are probably shared libraries.
> -	 */
> -	if (page_mapcount(page) != 1 && page_is_file_lru(page) &&
> -	    (vma->vm_flags & VM_EXEC))
> -		goto out;
> -
> -	/*
> -	 * Also do not migrate dirty pages as not all filesystems can move
> -	 * dirty pages in MIGRATE_ASYNC mode which is a waste of cycles.
> -	 */
> -	if (page_is_file_lru(page) && PageDirty(page))
> -		goto out;
> -
>  	isolated = numamigrate_isolate_page(pgdat, page);
>  	if (!isolated)
>  		goto out;