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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Alistair Popple , linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org, nouveau@lists.freedesktop.org, linux-pci@vger.kernel.org, linux-mm@kvack.org, linux-cxl@vger.kernel.org Subject: Re: [PATCH v4 1/7] mm/zone_device: Add order argument to folio_free callback Date: Mon, 12 Jan 2026 18:34:06 -0500 X-Mailer: MailMate (2.0r6290) Message-ID: <8DB7DC41-FDBD-4739-AABC-D363A1572ADD@nvidia.com> In-Reply-To: <20260112192816.GL745888@ziepe.ca> References: <20260111205820.830410-2-francois.dugast@intel.com> <874d29da-2008-47e6-9c27-6c00abbf404a@nvidia.com> <0D532F80-6C4D-4800-9473-485B828B55EC@nvidia.com> <20260112134510.GC745888@ziepe.ca> <218D42B0-3E08-4ABC-9FB4-1203BB31E547@nvidia.com> <20260112165001.GG745888@ziepe.ca> <86D91C8B-C3EA-4836-8DC2-829499477618@nvidia.com> <20260112182500.GI745888@ziepe.ca> <6AFCEB51-8EE1-4AC9-8F39-FCA561BE8CB5@nvidia.com> <20260112192816.GL745888@ziepe.ca> Content-Type: text/plain; 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No one >> should touch these free pages. Why bother resetting these states. > > ? that function does alot of stuff, thinks like uncharging the cgroup > should obviously happen at free time. > > What part of it are you looking at? page[1].flags.f &= ~PAGE_FLAGS_SECOND. It clears folio->order. free_tail_page_prepare() clears ->mapping, which is TAIL_MAPPING, and compound_head at the end. page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP. It clears PG_head for compound pages. These three parts undo prep_compound_page(). > >>> You can't refcount a folio out of nothing. It has to come from a >>> memory location that already is holding a refcount, and then you can >>> incr it. >> >> Right. There is also no guarantee that all code is correct and follows >> this. > > Let's concretely point at things that have a problem please. > >> My point here is that calling prep_compound_page() on a compound page >> does not follow core MM’s conventions. > > Maybe, but that doesn't mean it isn't the right solution.. In current nouveau code, ->free_folios is used holding the freed folio. In nouveau_dmem_page_alloc_locked(), the freed folio is passed to zone_device_folio_init(). If the allocated folio order is different from the freed folio order, I do not know how you are going to keep track of the rest of the freed folio. Of course you can implement a buddy allocator there. If this still does not convince you that overwriting an existing compound page with a different order configuration is a bad idea, feel free to do whatever you think it is right. Best Regards, Yan, Zi