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

[Baolin Wang]

On 8/21/2023 10:20 AM, Huang, Ying wrote:
> 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.

Indeed.

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

Sounds reasonable to me. Thanks for your input.

> --
> 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;