Re: bug: move_pages(2) does not udpate "status" if no pages are moved

[Felix Abecassis <fabecassis@nvidia.com>]

On Wed, Dec 04, 2019 at 12:17:59PM -0800, Yang Shi wrote:
> On Wed, Dec 4, 2019 at 11:01 AM Felix Abecassis <fabecassis@nvidia.com> wrote:
> >
> > Hello all,
> >
> > On kernel 5.3, when using the move_pages syscall (wrapped by libnuma) and all
> > pages happen to be on the right node already, this function returns 0 but the
> > "status" array is not updated. This array potentially contains garbage values
> > (e.g. from malloc(3)), and I don't see a way to detect this.
> >
> > Looking at the kernel code, we are probably exiting do_pages_move here:
> > out_flush:
> >     if (list_empty(&pagelist))
> >         return err;
> 
> May you please give the below patch a try? I just did build test.
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a8f87cb..f2f1279 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1517,7 +1517,8 @@ static int do_move_pages_to_node(struct mm_struct *mm,
>   * 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)
> + int node, struct list_head *pagelist, bool migrate_all,
> + int __user *status, int start)
>  {
>   struct vm_area_struct *vma;
>   struct page *page;
> @@ -1543,8 +1544,10 @@ static int add_page_for_migration(struct
> mm_struct *mm, unsigned long addr,
>   goto out;
> 
>   err = 0;
> - if (page_to_nid(page) == node)
> + if (page_to_nid(page) == node) {
> + err = store_status(status, start, node, 1);
>   goto out_putpage;
> + }
> 
>   err = -EACCES;
>   if (page_mapcount(page) > 1 && !migrate_all)
> @@ -1639,7 +1642,9 @@ static int do_pages_move(struct mm_struct *mm,
> nodemask_t task_nodes,
>   * report them via status
>   */
>   err = add_page_for_migration(mm, addr, current_node,
> - &pagelist, flags & MPOL_MF_MOVE_ALL);
> + &pagelist, flags & MPOL_MF_MOVE_ALL, status,
> + i);
> +
>   if (!err)
>   continue;
>

Thanks! I have tested this patch on top of 5.4-rc8, and I can comfirm it fixes
this particular test case.

> >
> > Here is a sample C program to reproduce the problem:
> > /* $ gcc move_pages_bug.c -lnuma -o move_pages_bug */
> >
> > #define _DEFAULT_SOURCE
> > #include <sys/mman.h>
> >
> > #include <numaif.h>
> > #include <stdlib.h>
> > #include <stdio.h>
> > #include <unistd.h>
> >
> > 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]);
> > }
> >
> > And here is a sample output showing the "garbage" 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
> >
> > Note that passing NULL as the "nodes" argument works as expected here.
> >
> > Also, it seems that it's the last "run-length" of pages on the right node(s)
> > that triggers this problem, e.g. if I add "nodes[0] = nodes[1] = 0", then the
> > output becomes:
> > $ ./move_pages_bug
> > move_pages: 0
> > status[0] = 0
> > status[1] = 0
> > status[2] = 208
> > status[3] = 208
> > status[4] = 208
> > status[5] = 208
> > status[6] = 208
> > status[7] = 208
> >
> > And with just "nodes[7] = 0;", the first run-length of pages gets assigned
> > correctly:
> > $ ./move_pages_bug
> > move_pages: 0
> > status[0] = 1
> > status[1] = 1
> > status[2] = 1
> > status[3] = 1
> > status[4] = 1
> > status[5] = 1
> > status[6] = 1
> > status[7] = 0
> >
> >
> > Thank you,
> >