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

>
> 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.
--
Michal Hocko
SUSE Labs

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