Re: [PATCH 0/2] mm/mempolicy: stop copying state in the interleave paths

[Andrew Morton]

On Fri, 28 Aug 2026 21:59:41 -0400 Gregory Price <gourry@xxxxxxxxxx> wrote:

> The interleave node selectors and bulk allocators take copies of
> nodemasks and node weights (for weighted interleave) in the fault path.
> Both of these copies can be entirely eliminated.
> 
> For node weights, use SRCU to pin the weights in place.  This eliminates
> a copy and a kmalloc from the bulk allocator path.
> 
> For nodemasks, we can operate directly on pol->nodes as long as we bounds
> check the walk.  A concurrent rebind can shrink the mask, or tear the read
> of it so the mask appears empty.
> 
>  - The interleave node selectors fall back to numa_node_id() when that
>    happens, which is what they already did when a copy came back empty.
> 
>  - The bulk allocator simply returns what it managed to allocate.
> 
> The node count and weight totals are read separately from the nodemask
> walk that consumes them - creating a time-of-check / time-of-use race.
> Just clamp the walk to a single pass (number of nodes), and clamp each
> bulk allocation chunk to the space left in the request.
> 
> The cost is distribution accuracy during a rebind.  The copies never
> corrected for that either - they only kept the code from dividing by
> zero and overrunning the allocation request.

Not very well, it seems.  Sashiko thinks there's a div-by-zero in
alloc_pages_bulk_interleave().

	https://sashiko.dev/#/patchset/20260829015943.1258774-1-gourry@xxxxxxxxxx

> Gregory Price (2):
>   mm/mempolicy: use SRCU for the weighted interleave state
>   mm/mempolicy: stop copying the nodemask in the interleave paths

Looks nice, thanks - I'll queue it for testing.