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From: Shakeel Butt To: Matthew Wilcox Cc: Vlastimil Babka , Johannes Weiner , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , linux-mm@kvack.org, Michal Hocko , Muchun Song , cgroups@vger.kernel.org Subject: Re: [PATCH v4 14/16] memcg: Convert mem_cgroup_from_obj_folio() to mem_cgroup_from_obj_slab() Message-ID: <72bs7m3juclmj6beuakzkoka2n5vej22qnrxnzsumqfkyfkk2r@gfoxlh4innwk> References: <20251113000932.1589073-1-willy@infradead.org> <20251113000932.1589073-15-willy@infradead.org> <20251113161424.GB3465062@cmpxchg.org> <45ea66a3-bf8b-4c12-89cd-d15ba26763fa@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 87820A0006 X-Stat-Signature: 8mg7y9rwtd6ghq5aw1t9sa1fwh9nhuzu X-Rspam-User: X-HE-Tag: 1763070857-162471 X-HE-Meta: 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 Bu/Uzamj 7wMW4t8PXk99zjeRpdDpIZEvS/kHqF9zk4MicU/79aRZBdGaeDQEMOazvzcYJ7p4P6OFE8rGh/1Ah2hEO5WeKN1+9L8bqzEj+6i3HhtgFBtk6NK88lYeHFQ0qXos2SlOC4vBb5mYkZZZQSTXxwZTSR4WB4cAU/gXs63SX/Tvgh12U3Ud4YaSnHIsK3mh5iHdAnLWtIuw7kZ1qMj+1dej4lngQgSQDu+pybv8muCcImc/Sx6GKLqL6sENttk3CR5DtIyPV 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 Thu, Nov 13, 2025 at 08:33:50PM +0000, Matthew Wilcox wrote: > On Thu, Nov 13, 2025 at 11:42:01AM -0800, Shakeel Butt wrote: > > On Thu, Nov 13, 2025 at 05:28:59PM +0100, Vlastimil Babka wrote: > > > > E.g. !vmap kernel stack pages -> mod_lruvec_kmem_state -> mem_cgroup_from_obj_slab > > > > > > > > How about: > > > > > > > > if ((slab = virt_to_slap(p))) > > > > return mem_cgroup_from_obj_slab(slab, p); > > > > return folio_memcg_check(virt_to_folio(p), p); > > > > > > page_memcg_check() maybe instead? we shouldn't get a tail page here, no? > > > > Do you mean page_memcg_check(virt_to_page(p), p)? But virt_to_page(p) > > can return tail page, right? > > Only if it's legitimate to call mod_lruvec_kmem_state() with "a pointer > to somewhere inside the object" rather than "a pointer to the object". > > For example, it's legitimate to call copy_to_user() with a pointer > somewhere inside the object, so the usercopy code has to handle that > case. But it's only legitimate to call kfree() with a pointer that's > at the start of an object, so kfree() can cheerfully BUG_ON() > PageTail(virt_to_page(p)). Yes it makes sense. Though for slab objects we are allowing to pass any address within the object (address of list_head for list_lru) to mem_cgroup_from_slab_obj() (and thus mod_lruvec_kmem_state()) but for normal kernel memory, if we use virt_to_page() then we will restrict to the starting address. Anyways just noticed this pecularity, not suggesting to do anything differently.