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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
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On 12/4/19 8:21 PM, 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<---

This is correct correct code, so:

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

...with a few nitpicky notes about comments, below, that might help:

>
> 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.
>
> 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.
>
> 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.
>
> mm/migrate.c | 36 ++++++++++++++++++++++++++++++------
> 1 file changed, 30 insertions(+), 6 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a8f87cb..f1090a0 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1512,17 +1512,21 @@ 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
> + * 0 - when it has been queued or the page doesn't need to be migrated
> + * because it is already on the target node
> + * 1 - if store_status() is failed


I recommend this wording instead:

* Returns:
* errno - if the page cannot be found/isolated
* 0 - when it has been queued or the page doesn't need to be migrated
* because it is already on the target node
* 1 - The page doesn't need to be migrated because it is already on the
* target node. However, attempting to store the node ID in the status
* array failed. Unlike other failures in this function, this case
* needs to turn into a fatal failure in the calling function.


> */
> 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;
> unsigned int follflags;
> int err;
> + bool same_node = false;
>
> down_read(&mm->mmap_sem);
> err = -EFAULT;
> @@ -1543,8 +1547,10 @@ 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) {
> + same_node = true;
> goto out_putpage;
> + }
>
> err = -EACCES;
> if (page_mapcount(page) > 1 && !migrate_all)
> @@ -1578,6 +1584,16 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
> put_page(page);
> out:
> up_read(&mm->mmap_sem);
> +
> + /*
> + * Must call store_status() after releasing mmap_sem since put_user
> + * need acquire mmap_sem too, otherwise potential deadlock may exist.
> + */
> + if (same_node) {
> + if (store_status(status, start, node, 1))
> + err = 1;
> + }
> +
> return err;
> }
>
> @@ -1639,10 +1655,18 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> * report them via status
> */

Let's change the comment above add_page_for_migration(), to read:

/*
* Most errors in the page lookup or isolation are not fatal
* and we simply report them via the status array. However,
* positive error values are fatal.
*/


> 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;
>
> + /* store_status() failed in add_page_for_migration() */

...and let's replace the above line, with the following:

/*
* Most errors in the page lookup or isolation are not fatal
* and we simply report them via the status array. However,
* positive error values are fatal.
*/


> + if (err > 0) {
> + err = -EFAULT;
> + goto out_flush;
> + }
> +
> err = store_status(status, i, err, 1);
> if (err)
> goto out_flush;
>

And with that, I think the comments help a little bit more, in reading
through the code.


thanks,
--
John Hubbard
NVIDIA

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