On 2/11/19 8:27 PM, Andrew Morton wrote:

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

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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...

Please, the next time include linux-api and people involved in the previous thread [1] into the CC list. Likely there should have been a Suggested-by: for Alexander as well.

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Ugh, what a mess.

I'm afraid it's even somewhat worse mess now.

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

There's no such commit, that sha was probably from linux-next. The patch is still in mmotm [1]. Luckily, I would say. Maybe Linus or some automation could run some script to check for bogus Fixes tags before accepting patches?

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.

Based on the non-existing sha, hopefully it wasn't backported anywhere, but maybe some AI did anyway. Ah, seems like it indeed made it as far as 4.9, as a fix for non-existing commit and without proper linux-api consideration :( I guess it's too late to revert it for 5.0. Hopefully the change is really safe and won't break anything, i.e. hopefully nobody was determining MAX_NUMNODES by increasing buffer size until get_mempolicy() stopped returning EINVAL. Or other problem in e.g. CRIU context.

What about the manpage? It says "The value specified by maxnode is less than the number of node IDs supported by the system." which could be perhaps applied both to nr_node_ids or MAX_NUMNODES. Or should we update it?

[1] https://lore.kernel.org/linux-mm/631c44cc-df2d-40d4-a537-d24864df0679@nvidia... [2] https://www.ozlabs.org/~akpm/mmotm/broken-out/include-linux-nodemaskh-use-nr...