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

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@nvidia.com Tested-by: Felix Abecassis fabecassis@nvidia.com Cc: John Hubbard jhubbard@nvidia.com Cc: Michal Hocko mhocko@suse.com Cc: Christoph Lameter cl@linux.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Mel Gorman mgorman@techsingularity.net Cc: stable@vger.kernel.org 4.17+ Signed-off-by: Yang Shi yang.shi@linux.alibaba.com

---

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.