linux-kernel - Re: [RFC PATCH 0/2] zswap: fix placement inversion in memory tiering systems

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Message-ID: <2759fa95d0071f3c5e33a9c6369f0d0bcecd76b7@linux.dev>
Date: Sat, 29 Mar 2025 19:53:23 +0000
From: "Yosry Ahmed" <yosry.ahmed@...ux.dev>
To: "Nhat Pham" <nphamcs@...il.com>, linux-mm@...ck.org
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, chengming.zhou@...ux.dev,
 sj@...nel.org, kernel-team@...a.com, linux-kernel@...r.kernel.org,
 gourry@...rry.net, willy@...radead.org, ying.huang@...ux.alibaba.com,
 jonathan.cameron@...wei.com, dan.j.williams@...el.com,
 linux-cxl@...r.kernel.org, minchan@...nel.org, senozhatsky@...omium.org
Subject: Re: [RFC PATCH 0/2] zswap: fix placement inversion in memory tiering
 systems

March 29, 2025 at 1:02 PM, "Nhat Pham" <nphamcs@...il.com> wrote:

> Currently, systems with CXL-based memory tiering can encounter the
> following inversion with zswap: the coldest pages demoted to the CXL
> tier can return to the high tier when they are zswapped out,
> creating memory pressure on the high tier.
> This happens because zsmalloc, zswap's backend memory allocator, does
> not enforce any memory policy. If the task reclaiming memory follows
> the local-first policy for example, the memory requested for zswap can
> be served by the upper tier, leading to the aformentioned inversion.
> This RFC fixes this inversion by adding a new memory allocation mode
> for zswap (exposed through a zswap sysfs knob), intended for
> hosts with CXL, where the memory for the compressed object is requested
> preferentially from the same node that the original page resides on.

I didn't look too closely, but why not just prefer the same node by default? Why is a knob needed?

Or maybe if there's a way to tell the "tier" of the node we can prefer to allocate from the same "tier"?