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Sat, 29 Mar 2025 15:13:31 -0700 (PDT) MIME-Version: 1.0 References: <20250329110230.2459730-1-nphamcs@gmail.com> <2759fa95d0071f3c5e33a9c6369f0d0bcecd76b7@linux.dev> In-Reply-To: <2759fa95d0071f3c5e33a9c6369f0d0bcecd76b7@linux.dev> From: Nhat Pham Date: Sat, 29 Mar 2025 15:13:20 -0700 X-Gm-Features: AQ5f1JoFtT9vRL_SzGvf34p_JQN2uIMkBVhEuTZsO1Din3fxRh7P9rjpu-oQRZ0 Message-ID: Subject: Re: [RFC PATCH 0/2] zswap: fix placement inversion in memory tiering systems To: Yosry Ahmed Cc: linux-mm@kvack.org, akpm@linux-foundation.org, hannes@cmpxchg.org, chengming.zhou@linux.dev, sj@kernel.org, kernel-team@meta.com, linux-kernel@vger.kernel.org, gourry@gourry.net, willy@infradead.org, ying.huang@linux.alibaba.com, jonathan.cameron@huawei.com, dan.j.williams@intel.com, linux-cxl@vger.kernel.org, minchan@kernel.org, senozhatsky@chromium.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 7323A40004 X-Rspamd-Server: rspam05 X-Rspam-User: X-Stat-Signature: 66fqztjwgh8a3dqg53kbxerpecnc6hpk X-HE-Tag: 1743286412-549758 X-HE-Meta: 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 /VabdwE6 vyofeIny3jd8+t+ZpWQj2lVlhne74oVOoNQfg/LP1dnz/ZK4PSz69HWp1Y27eQdJOuaBYjR0HH7wcJf+zgOdfyhkHwWQLl1jSNcgF0dhxJ9Mg9Lb97ApUTXPknfToOEoiupNHRXlfyAjZYXtKydOdDAxyhw1GdUu7oEKAy8K712a03ce8NRTyr0uEpcpltcMr4uXpkG4wbJHXXt5MqIsYjHWke2Nqu0JDeDHfJu1HkcXbvDEzkHBzZosWF+rT8CNwHmlVS687lX7jLQfWSV6YhpxZQh1GSoOOwZhRlhSJ0pqLpLlupmw04/RlurcVvt3Bvm1u4a8LWA1pWt1Np3ty0uYrGw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.042410, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, Mar 29, 2025 at 12:53=E2=80=AFPM Yosry Ahmed wrote: > > March 29, 2025 at 1:02 PM, "Nhat Pham" 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 defau= lt? Why is a knob needed? Good question, yeah the knob is to maintain the old behavior :) It might not be optimal, or even advisable, for all set up. For hosts with node-based memory tiering, then yeah it's a good idea in general, but I don't quite know how to have information about that from the kernel's perspective. > > Or maybe if there's a way to tell the "tier" of the node we can prefer to= allocate from the same "tier"? Is there an abstraction of the "tier" that we can use here?