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[173.79.56.208]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7c04e111a58sm257668385a.92.2025.02.09.22.00.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 09 Feb 2025 22:00:04 -0800 (PST) Date: Mon, 10 Feb 2025 01:00:02 -0500 From: Gregory Price To: "Harry (Hyeonggon) Yoo" <42.hyeyoo@gmail.com> Cc: Honggyu Kim , Byungchul Park , kernel_team@skhynix.com, Matthew Wilcox , lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org, linux-cxl@vger.kernel.org Subject: Re: [LSF/MM/BPF TOPIC] Restricting or migrating unmovable kernel allocations from slow tier Message-ID: References: <20250207072024.GA48419@system.software.com> Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Feb 10, 2025 at 11:33:47AM +0900, Harry (Hyeonggon) Yoo wrote: > > Premise: Some ZONE_NORMAL capacity exists on CXL memory > due to its large capacity. > What you actually need to show to justify increasing the complexity is (at least - but not limited to) 1) structures you want to migrate are harmful when placed on slow memory ex) Is `struct page` on slow mem actually harmful? - no data? ex) Are page tables on slow mem actually harmful? - known, yes. 2) The structures cannot be made to take up less space on local tier ex) struct page can be shrunk - do that first ex) huge-pages can be deployed - do that first 3) the structures take up sufficient space that it matters ex) struct page after shrunk might not - do that first ex) page tables with multi-sized huge pages may not - do that first 4) Making the structures migratable actually does something useful are `struct page` or page tables after #2 and #3 both: a) going through hot/cold phases enough to warrant being tiered b) hot enough for long enough that migration matters? You can probably actually (maybe?) collect data on this today - but you still have to contend with #2 and #3. > > I don't understand why we shouldn't introduce more kernel movable memory > if that turns out to be beneficial? > No one is going to stop research you want to do. I'm simply expressing that I think it's an ill-advised path to take. ~Gregory