Re: [PATCH] numa: Change get_mempolicy() to use nr_node_ids instead of MAX_NUMNODES

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 11 Feb 2019 10:02:45 -0800 <rcampbell@nvidia.com> wrote:

> From: Ralph Campbell <rcampbell@nvidia.com>
> 
> The system call, get_mempolicy() [1], passes an unsigned long *nodemask
> pointer and an unsigned long maxnode argument which specifies the
> length of the user's nodemask array in bits (which is rounded up).
> The manual page says that if the maxnode value is too small,
> get_mempolicy will return EINVAL but there is no system call to return
> this minimum value. To determine this value, some programs search
> /proc/<pid>/status for a line starting with "Mems_allowed:" and use
> the number of digits in the mask to determine the minimum value.
> A recent change to the way this line is formatted [2] causes these
> programs to compute a value less than MAX_NUMNODES so get_mempolicy()
> returns EINVAL.
> 
> Change get_mempolicy(), the older compat version of get_mempolicy(), and
> the copy_nodes_to_user() function to use nr_node_ids instead of
> MAX_NUMNODES, thus preserving the defacto method of computing the
> minimum size for the nodemask array and the maxnode argument.
> 
> [1] http://man7.org/linux/man-pages/man2/get_mempolicy.2.html
> [2] https://lore.kernel.org/lkml/1545405631-6808-1-git-send-email-longman@redhat.com
> 

Ugh, what a mess.

For a start, that's a crazy interface.  I wish that had been brought to
our attention so we could have provided a sane way for userspace to
determine MAX_NUMNODES.

Secondly, 4fb8e5b89bcbbb ("include/linux/nodemask.h: use nr_node_ids
(not MAX_NUMNODES) in __nodemask_pr_numnodes()") introduced a
regession.  The proposed get_mempolicy() change appears to be a good
one, but is a strange way of addressing the regression.  I suppose it's
acceptable, as long as this change is backported into kernels which
have 4fb8e5b89bcbbb.