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[34.124.168.21]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-392f8893496sm3642752a91.2.2026.08.12.05.46.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Aug 2026 05:46:49 -0700 (PDT) Date: Wed, 12 Aug 2026 12:46:43 +0000 From: Pranjal Shrivastava To: Pratyush Yadav Cc: Mike Rapoport , Pasha Tatashin , Alexander Graf , Samiullah Khawaja , David Matlack , kexec@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 2/2] kho: Introduce preserve/restore APIs for high-order pages Message-ID: References: <20260803113944.3694290-1-praan@google.com> <20260803113944.3694290-3-praan@google.com> <2vxzmrur3880.fsf@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2vxzmrur3880.fsf@kernel.org> X-Rspamd-Queue-Id: B2FDE18000C X-Stat-Signature: pwjyskdzs6r5r3u8npjoqbtc5bdg5gm3 X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1786538811-15021 X-HE-Meta: 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 R4bdDKWx QHTqqNJ9lfGeZNu540hKFXxW5ujRDQaYrWEB/RTBIcWa8ITArXBFplArSNTw6BI6qvx97pZsyUpo6VIIErkMco7JEOJ2io52PrVcQTGeum/JYqxs27l9fGiMpe7Z3IFXwEH4WDiBGHDOccZdHkTLNvXhgT13O70F2iYJcNcH2OCKKzTuqSyfPlpsI/TUTRU708lZmStXg+EMldjer1xy37zt1iZOJ1Zxfw2gDe8YhSfgl2fWQ5hVngwxylJUwi1lkM4GcO6EQla3btwSBwpA0W4oPW1J5Ae2YA5itd6ARom4w6qZKacY8C5VVvX0s0H89uXzK/Im9ACzJvQPKt3czohCMesrlHaRlanAK Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Aug 12, 2026 at 01:10:23PM +0200, Pratyush Yadav wrote: > On Mon, Aug 03 2026, Pranjal Shrivastava wrote: > > > The current KHO page preservation APIs (e.g. kho_preserve_pages) assume > > that multi-page blocks are split into independent 4KB pages during > > restoration. This is incompatible with high-order non-compound pages, > > such as DMA buffers, which must be restored with tail pages having a > > zero reference count. > > > > Introduce explicit preserve and restore APIs for high-order pages, > > which preserve and restore a high-order page block as a single unit, > > applying a refcount of 1 to the head page while leaving tail pages at 0. > > Rename the existing internal helper to __kho_restore_page() and > > consolidate the common restoration code into it. > > > > Signed-off-by: Pranjal Shrivastava > > The code here looks very convoluted TBH. I think it will be simpler to > make kho_restore_page() only return non-compound pages. That is, it > returns 0 or higher order non-compound page. > > Then kho_restore_pages() can call kho_restore_page() and then do > split_page() on the page it got to turn it into 0-order pages. > kho_restore_folio() can call kho_restore_page() and then do > prep_compound_page() on the page it got. > > And then you expose kho_restore_page() to be used by DMA APIs to get > non-compound high order pages directly. > > How does that sound? > I like the suggestion to use a single base primitive and mold it via split_page() and prep_compound_page(). However, To implement this, we'll need __kho_restore_page(phys, &order) as the internal base primitive because callers (like kho_restore_folio) need to retrieve the order from the KHO metadata to pass into prep_compound_page(), and a raw non-compound struct page doesn't store its order. By having __kho_restore_page() initialize the head_ref = 1 & all_tails =0 refcount pattern by default, all three public APIs can just wrap it and call the appropriate MM subsystem helpers. I'll spin v5 with this Thanks, Praan