Re: [v2 PATCH] mm: move_pages: return valid node id in status if the page is already on the target node

[Yang Shi]

On 12/5/19 3:31 AM, Michal Hocko wrote:
> On Thu 05-12-19 12:21:18, Yang Shi wrote:
> > Felix Abecassis reports move_pages() would return random status if the
> > pages are already on the target node by the below test program:
> >
> > ---8<---
> >
> > int main(void)
> > {
> > 	const long node_id = 1;
> > 	const long page_size = sysconf(_SC_PAGESIZE);
> > 	const int64_t num_pages = 8;
> >
> > 	unsigned long nodemask =  1 << node_id;
> > 	long ret = set_mempolicy(MPOL_BIND, &nodemask, sizeof(nodemask));
> > 	if (ret < 0)
> > 		return (EXIT_FAILURE);
> >
> > 	void **pages = malloc(sizeof(void*) * num_pages);
> > 	for (int i = 0; i < num_pages; ++i) {
> > 		pages[i] = mmap(NULL, page_size, PROT_WRITE | PROT_READ,
> > 				MAP_PRIVATE | MAP_POPULATE | MAP_ANONYMOUS,
> > 				-1, 0);
> > 		if (pages[i] == MAP_FAILED)
> > 			return (EXIT_FAILURE);
> > 	}
> >
> > 	ret = set_mempolicy(MPOL_DEFAULT, NULL, 0);
> > 	if (ret < 0)
> > 		return (EXIT_FAILURE);
> >
> > 	int *nodes = malloc(sizeof(int) * num_pages);
> > 	int *status = malloc(sizeof(int) * num_pages);
> > 	for (int i = 0; i < num_pages; ++i) {
> > 		nodes[i] = node_id;
> > 		status[i] = 0xd0; /* simulate garbage values */
> > 	}
> >
> > 	ret = move_pages(0, num_pages, pages, nodes, status, MPOL_MF_MOVE);
> > 	printf("move_pages: %ld\n", ret);
> > 	for (int i = 0; i < num_pages; ++i)
> > 		printf("status[%d] = %d\n", i, status[i]);
> > }
> > ---8<---
> >
> > Then running the program would return nonsense status values:
> > $ ./move_pages_bug
> > move_pages: 0
> > status[0] = 208
> > status[1] = 208
> > status[2] = 208
> > status[3] = 208
> > status[4] = 208
> > status[5] = 208
> > status[6] = 208
> > status[7] = 208
> >
> > This is because the status is not set if the page is already on the
> > target node, but move_pages() should return valid status as long as it
> > succeeds.  The valid status may be errno or node id.
> >
> > We can't simply initialize status array to zero since the pages may be
> > not on node 0.  Fix it by updating status with node id which the page is
> > already on.  And, it looks we have to update the status inside
> > add_page_for_migration() since the page struct is not available outside
> > it.
> The code is indeed more complex than I wanted but I couldn't figure an
> easier way back then. I wanted to keep store_status at a single place
> because the failure handling is quite complex already.
>
> > Make add_page_for_migration() return 1 if store_status() is failed in
> > order to not mix up the status value since -EFAULT is also a valid
> > status.
> Can we simply return 1 when there is something to migrate instead?
> Something like
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 4fe45d1428c8..f3730804b8d4 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1516,9 +1516,9 @@ static int do_move_pages_to_node(struct mm_struct *mm,
>   /*
>    * Resolves the given address to a struct page, isolates it from the LRU and
>    * puts it to the given pagelist.
> - * Returns -errno if the page cannot be found/isolated or 0 when it has been
> - * queued or the page doesn't need to be migrated because it is already on
> - * the target node
> + * Returns -errno if the page cannot be found/isolated or 0 when it doesn't have
> + * to be migrate or 1 dwhen it has been queued or the page doesn't need to be
> + * migrated because it is already on the target node
>    */
>   static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
>   		int node, struct list_head *pagelist, bool migrate_all)
> @@ -1557,7 +1557,7 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
>   	if (PageHuge(page)) {
>   		if (PageHead(page)) {
>   			isolate_huge_page(page, pagelist);
> -			err = 0;
> +			err = 1;
>   		}
>   	} else {
>   		struct page *head;
> @@ -1567,7 +1567,7 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
>   		if (err)
>   			goto out_putpage;
>   
> -		err = 0;
> +		err = 1;
>   		list_add_tail(&head->lru, pagelist);
>   		mod_node_page_state(page_pgdat(head),
>   			NR_ISOLATED_ANON + page_is_file_cache(head),
> @@ -1644,8 +1644,14 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>   		 */
>   		err = add_page_for_migration(mm, addr, current_node,
>   				&pagelist, flags & MPOL_MF_MOVE_ALL);
> -		if (!err)
> +		if (!err) {
> +			err = store_status(status, i, current_node, 1);
> +			if (err)
> +				goto out_flush;
> +			continue;
> +		} else if (err > 0) {
>   			continue;
> +		}
>   
>   		err = store_status(status, i, err, 1);
>   		if (err)
>
> this would still keep store_status ugliness at a single place.

Thanks for the suggestion, it looks neater, will do in v2.

> > Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
> > Reported-by: Felix Abecassis <fabecassis@xxxxxxxxxx>
> > Tested-by: Felix Abecassis <fabecassis@xxxxxxxxxx>
> > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx> 4.17+
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> > v2: *Correted the return value when add_page_for_migration() returns 1.
> >
> > John noticed another return value inconsistency between the implementation and
> > the manpage.  The manpage says it should return -ENOENT if the page is already
> > on the target node, but it doesn't.  It looks the original code didn't return
> > -ENOENT either, I'm not sure if this is a document issue or not.  Anyway this
> > is another issue, once we confirm it we can fix it later.
> I do not remember all the details but my recollection is that there were
> several inconsistencies present before I touched the code and I've
> decided to not touch them without a clear usecase.

I agree. It looks nobody complained those inconsistency, and I didn't 
see LTP covers those.