Re: [RFC v2 4/5] mm/migrate: Create move_phys_pages syscall

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Tue, 19 Sep 2023 19:09:07 -0400
Gregory Price <gourry.memverge@gmail.com> wrote:

> Similar to the move_pages system call, instead of taking a pid and
> list of virtual addresses, this system call takes a list of physical
> addresses.
> 
> Because there is no task to validate the memory policy against, each
> page needs to be interrogated to determine whether the migration is
> valid, and all tasks that map it need to be interrogated.
> 
> This is accomplished in via a rmap_walk on the folio containing
> the page, and an interrogation of all tasks that map the page (by
> way of each task's vma).
> 
> Each page must be interrogated individually, which should be
> considered when using this to migrate shared regions.
> 
> The remaining logic is the same as the move_pages syscall. Some
> minor changes to do_pages_move are made (to check whether an
> mm_struct is passed) in order to re-use the existing migration code.
> 
> Signed-off-by: Gregory Price <gregory.price@memverge.com>

Hi Gregory

A few comments inline.

Thanks,

Jonathan


> diff --git a/mm/migrate.c b/mm/migrate.c
> index 1123d841a7f1..2d06557c0b80 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2165,9 +2165,118 @@ static int move_pages_and_store_status(int node,
>  	return store_status(status, start, node, i - start);
>  }
>  
> +struct rmap_page_ctxt {
> +	bool found;
> +	bool migratable;
> +	bool node_allowed;
> +	int node;
> +};
> +
> +/*
> + * Walks each vma mapping a given page and determines if those
> + * vma's are both migratable, and that the target node is within
> + * the allowed cpuset of the owning task.
> + */
> +static bool phys_page_migratable(struct folio *folio,
> +				 struct vm_area_struct *vma,
> +				 unsigned long address,
> +				 void *arg)
> +{
> +	struct rmap_page_ctxt *ctxt = (struct rmap_page_ctxt *)arg;

arg is a void * so no need for explicit cast.

> +	struct task_struct *owner = vma->vm_mm->owner;
> +	/* On non-memcg systems, the allowed set is the possible set */
> +#ifdef CONFIG_MEMCG
> +	nodemask_t task_nodes = cpuset_mems_allowed(owner);
> +#else
> +	nodemask_t task_nodes = node_possible_map;
> +#endif
> +
> +	ctxt->found |= true;

Given you or it with true, you might as well just set it..
	ctxt->found = true;
ends up the same.

> +	ctxt->migratable &= vma_migratable(vma);
> +	ctxt->node_allowed &= node_isset(ctxt->node, task_nodes);
> +
> +	return ctxt->migratable && ctxt->node_allowed;
> +}
> +

> +/*
> + * Validates the physical address is online and migratable.  Walks the folio
> + * containing the page to validate the vma is migratable and the cpuset node
> + * restrictions.  Then calls add_page_for_migration to isolate it from the
> + * LRU and place it into the given pagelist.
> + * Returns:
> + *     errno - if the page is not online, migratable, or can't be isolated
> + *     0 - when it doesn't have to be migrated because it is already on the
> + *         target node
> + *     1 - when it has been queued
> + */
> +static int add_phys_page_for_migration(const void __user *p, int node,
> +				       struct list_head *pagelist,
> +				       bool migrate_all)
> +{
> +	unsigned long pfn;
> +	struct page *page;
> +	struct folio *folio;
> +	int err;
> +	struct rmap_page_ctxt rmctxt = {
> +		.found = false,
> +		.migratable = true,
> +		.node_allowed = true,
> +		.node = node
> +	};
> +	struct rmap_walk_control rwc = {
> +		.rmap_one = phys_page_migratable,
> +		.arg = &rmctxt
> +	};
> +
> +	pfn = ((unsigned long)p) >> PAGE_SHIFT;
> +	page = pfn_to_online_page(pfn);
> +	if (!page || PageTail(page))
> +		return -ENOENT;
> +
> +	folio = phys_migrate_get_folio(page);
> +	if (folio)
> +		rmap_walk(folio, &rwc);
Why not just tidy up the !folio case more directly as if
it's not we only get to the first condition as rmctxt.found == false
anyway, so this is the same as.

	if (!folio)
		return -ENOENT.

	rmap_walk(foil, &rwc);


> +
> +	if (!rmctxt.found)
> +		err = -ENOENT;
> +	else if (!rmctxt.migratable)
> +		err = -EFAULT;
> +	else if (!rmctxt.node_allowed)
> +		err = -EACCES;
> +	else
> +		err = add_page_for_migration(page, node, pagelist, migrate_all);
> +
> +	if (folio)

With above changes this condition can go.

> +		folio_put(folio);
> +
> +	return err;
> +}
> +
>  /*
>   * Migrate an array of page address onto an array of nodes and fill
>   * the corresponding array of status.
> + *
> + * When the mm argument is not NULL, task_nodes is expected to be the
> + * cpuset nodemask for the task which owns the mm_struct, and the
> + * values located in (*pages) are expected to be virtual addresses.
> + *
> + * When the mm argument is NULL, the values located at (*pages) are
> + * expected to be physical addresses, and task_nodes is expected to
> + * be empty.
>   */
>  static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>  			 unsigned long nr_pages,
> @@ -2181,6 +2290,10 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>  	int start, i;
>  	int err = 0, err1;
>  
> +	/* This should never occur in regular operation */
> +	if (!mm && nodes_weight(task_nodes) > 0)

You document below that task_nodes isn't used if !mm but it is used
to indicate this condition...

> +		return -EINVAL;
> +
>  	lru_cache_disable();
>  
>  	for (i = start = 0; i < nr_pages; i++) {
> @@ -2209,7 +2322,14 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>  			goto out_flush;
>  
>  		err = -EACCES;
> -		if (!node_isset(node, task_nodes))
> +		/*
> +		 * if mm is NULL, then the pages are addressed via physical
> +		 * address and the task_nodes structure is empty. Validation
> +		 * of migratability is deferred to add_phys_page_for_migration
> +		 * where vma's that map the address will have their node_mask
> +		 * checked to ensure the requested node bit is set.
> +		 */
> +		if (mm && !node_isset(node, task_nodes))
>  			goto out_flush;
>  

> @@ -2317,6 +2444,37 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
>  	mmap_read_unlock(mm);
>  }
>  
> +/*
> + * Determine the nodes pages pointed to by the physical addresses in the
> + * pages array, and store those node values in the status array
> + */
> +static void do_phys_pages_stat_array(unsigned long nr_pages,
> +				     const void __user **pages, int *status)
> +{
> +	unsigned long i;
> +
> +	for (i = 0; i < nr_pages; i++) {
> +		unsigned long pfn = (unsigned long)(*pages) >> PAGE_SHIFT;
> +		struct page *page = pfn_to_online_page(pfn);
> +		int err = -ENOENT;
> +
> +		if (!page)
> +			goto set_status;
> +
> +		get_page(page);
> +
> +		if (!is_zone_device_page(page))
> +			err = page_to_nid(page);
> +
> +		put_page(page);
> +set_status:
> +		*status = err;

Given you update it unconditionally this seems to wipe out earlier
potential errors with the nid.

> +
> +		pages++;
> +		status++;
> +	}
> +}

>  
> +/*
> + * Move a list of physically-addressed pages to the list of target nodes
> + */
> +static int kernel_move_phys_pages(unsigned long nr_pages,
> +				  const void __user * __user *pages,
> +				  const int __user *nodes,
> +				  int __user *status, int flags)
> +{
> +	int err;
> +	nodemask_t dummy_nodes;
> +
> +	if (flags & ~(MPOL_MF_MOVE|MPOL_MF_MOVE_ALL))

Spaces around the |

> +		return -EINVAL;
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +
> +	/*
> +	 * When the mm argument to do_pages_move is null, the task_nodes
> +	 * argument is ignored, so pass in an empty nodemask as a dummy.

If ignored, why bother clearing it?  Looks like you are using it as
as signal rather than ignoring it. So not sure this comment is correct.

> +	 */
> +	nodes_clear(dummy_nodes);
> +	if (nodes)
> +		err = do_pages_move(NULL, dummy_nodes, nr_pages, pages,
> +			nodes, status, flags);
> +	else
> +		err = do_pages_stat(NULL, nr_pages, pages, status);
> +
> +	return err;

If nothing to do, just return in the two branches and loose the local variable err.


> +}
> +
> +SYSCALL_DEFINE5(move_phys_pages, unsigned long, nr_pages,
> +		const void __user * __user *, pages,
> +		const int __user *, nodes,
> +		int __user *, status, int, flags)
> +{
> +	return kernel_move_phys_pages(nr_pages, pages, nodes, status, flags);
> +}
> +
Looks like 1 blank line enough to me...

> +
>  #ifdef CONFIG_NUMA_BALANCING