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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1rhYUb-00000004shb-12OD; Tue, 05 Mar 2024 17:22:17 +0000 Date: Tue, 5 Mar 2024 17:22:17 +0000 From: Matthew Wilcox To: Yu Zhao Cc: lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org, Jonathan Corbet Subject: Re: [Chapter One] THP zones: the use cases of policy zones Message-ID: References: <20240229183436.4110845-1-yuzhao@google.com> <20240229183436.4110845-2-yuzhao@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 66061C0005 X-Stat-Signature: bfjpbnh713rpbz3w1kej3953uxschdeb X-Rspam-User: X-HE-Tag: 1709659343-607108 X-HE-Meta: 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 Kk6q6iaD q6x7Dee71w4EBReLAUF+wSCylLDEMH2Tfd2AxWjQ3hTqlDemFUIea1bfCz6Go7kVCFg2z5APA/7WdITgmb4XtSn+MiRyb5/yXOo3DaKCCFQKmKO3J9NtzixFoPD8xNLEPcMmk4gsXf4K+gtcCjMVSZIXJWZXKW2GQc96Afp7REjRf+tOhgG59lFI12et8BTsmr7N/ X-Bogosity: Ham, tests=bogofilter, spamicity=0.000082, 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 Mon, Mar 04, 2024 at 03:19:42PM +0000, Matthew Wilcox wrote: > On Thu, Feb 29, 2024 at 11:34:33AM -0700, Yu Zhao wrote: > > ZONE_MOVABLE becomes the first policy zone, followed by two new policy > > zones: > > 1. ZONE_NOSPLIT, which contains pages that are movable (inherited from > > ZONE_MOVABLE) and restricted to a minimum order to be > > anti-fragmentation. The latter means that they cannot be split down > > below that order, while they are free or in use. > > 2. ZONE_NOMERGE, which contains pages that are movable and restricted > > to an exact order. The latter means that not only is split > > prohibited (inherited from ZONE_NOSPLIT) but also merge (see the > > reason in Chapter Three), while they are free or in use. > > These two zones end up solving a problem for memdescs. So I'm in favour! > I added Option 5 to https://kernelnewbies.org/MatthewWilcox/BuddyAllocator I realised that we don't even need a doubly-linked-list for ZONE_NOMERGE (would ZONE_FIXEDSIZE be a better name?) We only need a oubly linked list to make removal from the middle of the list an O(1) operation, and we only remove from the middle of a list when merging. So we can simply keep a stack of free "pages" and we have 60 bits to point to the next memdesc, so we can easily cover all memory that can exist in a 64-bit machine in ZONE_NOMERGE. ZONE_NOSPLIT would be limited to the first 1PB of memory (assuming it has a minimum size of 2MB -- with 29 bits to refer to each of next & prev, 29 + 21 = 50 bits of address space).