Re: [PATCH v2] mm/page_alloc: only skip kswapd nodes below their kswapd watermark

[Joshua Hahn]

On Thu, Nov 12, 2026 at 01:18:30AM -0500, Gregory Price (Meta) wrote:
> On 2026-11-10 14:44 -0800, Joshua Hahn wrote:
> > Commit 19c5fb83f2a4 ("mm: page_alloc: avoid kswapd thrashing due to
> > NUMA restrictions") makes get_page_from_freelist() skip any node with an
> > active kswapd on the first pass through the zonelist, so unrestricted
> > allocations don't immediately consume the memory kswapd is freeing.
> > 
> > But kswapd is also woken for high-order requests. With THP defrag set to
> > defer, each THP fault that fails on a fragmented node wakes kswapd at
> > PMD order. kswapd reclaims a batch, wakes kcompactd, and sleeps until
> > the next failed fault, so it stays awake much of the time even on a node
> > with plenty of free order-0 memory. This means those order-0 allocations
> > get directed elsewhere.
> > 
> > This was observed on some Meta production workloads running on 2-NUMA
> > systems, where processes with a preference for node0 allocations
> > continuously allocated order-0 pages on node1, despite node0 having
> > hundreds of GB of free memory.
> > 
> > Only skip a zone with an active kswapd if it is also below kswapd's
> > target watermark.
> >
> 
> I feel this commit message could have been simplified further.
> Brevity is the soul of wit.
> 
>   patch (details) skips any node with kswapd running - assuming that
>   this means memory is low, but kswapd is woken for other reasons (e.g.
>   order-N allocation failures).
> 
>   This results in fallback allocations under X conditions, which leads
>   to xyz performance isues.
> 
>   Only skip nodes with kswap running if the node is under the relevant
>   low-memory watermark.
> 
>   (test details)

Thanks Gregory,

I was going to ask to have the commit message updated as a fixlet but
I think given the amount of change going in I will just send a new
version with a more concise commit message to make it less confusing.

Thanks again, have a great day!
Joshua

> Otherwise this looks good to me.  Looking forward to the watermark
> cleanup afterwards - i agree the change here is a bit ugly, but
> probably preferable for the backport.
> 
> Acked-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> 
> -- 
> ~Gregory
>