Re: [PATCH v1 2/2] osdep: support mempolicy for preallocation in os_mem_prealloc

[David Hildenbrand <david@redhat.com>]

On 07.12.21 08:06, Daniil Tatianin wrote:
> This is needed for cases where we want to make sure that a shared memory
> region gets allocated from a specific NUMA node. This is impossible to do
> with mbind(2) because it ignores the policy for memory mapped with
> MAP_SHARED. We work around this by calling set_mempolicy from prealloc
> threads instead.

That's not quite true I think, how were you able to observe this? Do you
have a reproducer?

While the man page says:

"
The specified policy will be ignored for any  MAP_SHARED  mappings  in
the  specified  memory range. Rather  the pages will be allocated
according to the memory policy of the thread that caused the page to be
allocated.  Again, this may not be the thread that called mbind().
"

What it really means is that as long as we access that memory via the
*VMA* for which we called mbind(), which is the case when *not* using
fallocate() to preallocate memory, we end up using the correct policy.


I did experiments a while ago with hugetlbfs shared memory and it
properly allocated from the right node. So I'd be curious how you
trigger this.

-- 
Thanks,

David / dhildenb