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[96.255.20.138]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-894d36a9bdbsm6298466d6.11.2026.01.27.15.44.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Jan 2026 15:44:01 -0800 (PST) Date: Tue, 27 Jan 2026 18:43:58 -0500 From: Gregory Price To: Bing Jiao Cc: Akinobu Mita , linux-cxl@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, hannes@cmpxchg.org, david@kernel.org, mhocko@kernel.org, zhengqi.arch@bytedance.com, shakeel.butt@linux.dev, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com Subject: Re: [PATCH v3 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier Message-ID: References: <20260108101535.50696-1-akinobu.mita@gmail.com> <20260108101535.50696-4-akinobu.mita@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 511911A0008 X-Stat-Signature: de6ijy15pqy7h7y581hsjjwq4mnze4g4 X-HE-Tag: 1769557443-657420 X-HE-Meta: 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 kItWtFrQ XtB+DmNS/SuBB0PjADagrvQlBgl58kNf53m7AtKHOQ6wa/Atg1K+Spu9nyVfbH4wjWgYOR7yFOV6LJeWJDlNSviT6Y9/pRkxD8zgbHbEbK2dW5RdgoazHw4FW5CEu5F1Hso0aFtTgjKC4XkLgi39lsBbCIZcfe98bz8ZHxLvXTj69xQeaW+woq4YEk31wxY6hetqqQPzPxO7I7enRDHa4GZ+D1a11jPSi4fxw8ShplH4H4IYwEzJdaChf5ZkdtpOi/iUf+TDLdSGE0bumdPpmrp4Wpr0oPt3H/QYmPv71NhfiOkOscC1wC48dDTXtmIRfsps+YxIoxPgjDJacAAo9Fml+NohTAIoycUPxB8QtcVPB4FKT4brSukUTlrVUiN/iKDmPZW5VLKEMnhikRKS6265/I7I8X7YC0B5jZOwyOB4NAv8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 Tue, Jan 27, 2026 at 11:28:59PM +0000, Bing Jiao wrote: > Hi, Gregory, hope you are doing well. > > I observed that during the allocation of a large folio, > alloc_migration_target() cleans __GFP_RECLAIM but subsequently applies > GFP_TRANSHUGE. Given that GFP_TRANSHUGE includes __GFP_DIRECT_RECLAIM, > I am wondering if this triggers a form of reclamation that should be > avoided during demotion. > > struct folio *alloc_migration_target(struct folio *src, unsigned long private) > ... > if (folio_test_large(src)) { > /* > * clear __GFP_RECLAIM to make the migration callback > * consistent with regular THP allocations. > */ > gfp_mask &= ~__GFP_RECLAIM; > gfp_mask |= GFP_TRANSHUGE; > order = folio_order(src); > } > > #define GFP_TRANSHUGE (GFP_TRANSHUGE_LIGHT | __GFP_DIRECT_RECLAIM) > I think the answer is that the demotion code is a mess and no one actually knows what it's doing. We probably need to rework this entirely because we've now found at least 2 paths which clean and then add reclaim. ~Gregory