[Bug 217253] New: mbind, set_mempolicy, migrate_pages: maxnode description is off-by-one

[Bug 217253] New: mbind, set_mempolicy, migrate_pages: maxnode description is off-by-one

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=217253

            Bug ID: 217253
           Summary: mbind, set_mempolicy, migrate_pages: maxnode
                    description is off-by-one
           Product: Documentation
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: man-pages
          Assignee: documentation_man-pages@kernel-bugs.osdl.org
          Reporter: tonyb@cybernetics.com
        Regression: No

linux/mm/mempolicy.c::get_nodes() does "--maxnode" at the beginning, so:
maxnode == 0 is invalid (-EINVAL).
maxnode == 1 specifies the empty set of nodes (the man pages currently say to
use maxnode == 0).
maxnode == 2 indicates one valid bit in nodemask.
maxnode == 3 indicates two valid bits in nodemask.
etc.

Incorrect section from mbind manpage:

"nodemask points to a bit mask of nodes containing up to maxnode bits.  The bit
mask size is rounded to the next multiple of sizeof(unsigned long), but the
kernel will use bits only up to maxnode.  A NULL value of nodemask or a maxnode
value of zero specifies the empty set of nodes.  If the value of maxnode is
zero, the nodemask argument is ignored."

I am not sure if this was an intentional design choice or a bug that got
enshrined in the userspace API, but userspace programs "in the know" seem to
rely on this now:

https://gitlab.com/qemu-project/qemu/-/blob/60ca584b8af0de525656f959991a440f8c191f12/backends/hostmem.c#L369

Also, the commit message for linux commit c6018b4b2549 ("mm/mempolicy: add
set_mempolicy_home_node syscall") shows using "new_nodes->size + 1", so this
API bug/choice seems to be known within the kernel community.

Here is a related bugzilla entry that treats the problem as a kernel bug rather
than a documentation issue:
https://bugzilla.kernel.org/show_bug.cgi?id=201433

But since "fixing" the bug (assuming that it was unintentional) might break
existing userspace programs that work around the bug, I suggest fixing the
documentation instead.  But that is just my opinion as a user who just ran into
the bug and did some investigating; best to check with the kernel maintainers
for their opinion.

Related:
linux commit 050c17f239fd ("numa: change get_mempolicy() to use nr_node_ids
instead of MAX_NUMNODES") commit message talks about calculating maxnode for
get_mempolicy().

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[Bug 217253] mbind, set_mempolicy, migrate_pages: maxnode description is off-by-one

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=217253

--- Comment #1 from Alejandro Colomar (alx@kernel.org) ---
Thanks for the investigation.  I CCed the maintainer.  If you have any
specific suggestions for fixing the documentation, would you mind
preparing a patch according to the ./CONTRIBUTING file in the
man-pages repository?

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[Bug 217253] mbind, set_mempolicy, migrate_pages: maxnode description is off-by-one

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=217253

Alejandro Colomar (alx@kernel.org) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |NEEDINFO

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