On 20.07.24 19:35, Jerome Glisse wrote:
Because maxnode bug there is no way to bind or migrate_pages to the
last node in multi-node NUMA system unless you lie about maxnodes
when making the mbind, set_mempolicy or migrate_pages syscall.
Manpage for those syscall describe maxnodes as the number of bits in
the node bitmap ("bit mask of nodes containing up to maxnode bits").
Thus if maxnode is n then we expect to have a n bit(s) bitmap which
means that the mask of valid bits is ((1 << n) - 1). The get_nodes()
decrement lead to the mask being ((1 << (n - 1)) - 1).
The three syscalls use a common helper get_nodes() and first things
this helper do is decrement maxnode by 1 which leads to using n-1 bits
in the provided mask of nodes (see get_bitmap() an helper function to
get_nodes()).
The lead to two bugs, either the last node in the bitmap provided will
not be use in either of the three syscalls, or the syscalls will error
out and return EINVAL if the only bit set in the bitmap was the last
bit in the mask of nodes (which is ignored because of the bug and an
empty mask of nodes is an invalid argument).
I am surprised this bug was never caught ... it has been in the kernel
since forever.
Let's look at QEMU: backends/hostmem.c
/*
* We can have up to MAX_NODES nodes, but we need to pass maxnode+1
* as argument to mbind() due to an old Linux bug (feature?) which
* cuts off the last specified node. This means backend->host_nodes
* must have MAX_NODES+1 bits available.
*/
Which means that it's been known for a long time, and the workaround
seems to be pretty easy.
So I wonder if we rather want to update the documentation to match reality.
--
Cheers,
David / dhildenb
