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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="VYQ/jJ6O"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 00E31606A327 for ; Tue, 3 Nov 2026 17:04:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=VYQ/jJ6O Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1C21D1F000C1; Tue, 3 Nov 2026 17:04:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793725446; bh=bEnfqIn1d5BmxBfGTOzDtpPMnd7ZSc057uWCU5mMqxc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=VYQ/jJ6OVQ6W1XY81dONcqltFtpUGJBaOQM0hKv7qY0ir4Sh4jNP4dSLNxeb3Bn24 3Gv3g//UQ3kMbSr3HY4r+ndXng9fr0XXATyj1QaloIIbhkEzR5HTkmrP4Vcse8kpo0 RTB2wy5BbMTchXYcF7zXOxaO2izeHvjallY/LhVKnRgY4MCXgiOreh4DHikVTYMIH7 xIrQkanKaPV9qzCyiorwidm/pPyKmNuKQzY8w74z8WHp2+YI54x87sOYTLGwXlsu6W 4akm0drymwFocuJZNvWJsm6RYg4z+p1f++nr8BKhQBJ+tBiXfTe4G0FjPzTupcUcSS h6TlpN/m9hUIg== Message-ID: Date: Tue, 3 Nov 2026 18:04:01 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/8] mm/gup: break out gup_fill_pages() helper To: Rik van Riel , linux-kernel@vger.kernel.org Cc: Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu , Suren Baghdasaryan , Yuan-Hao Hsu , Christoph Hellwig , linux-mm@kvack.org References: <20261102193026.3860700-1-riel@surriel.com> <20261102193026.3860700-2-riel@surriel.com> <3a7d12b52a5de61277842f2cca0dd90d42728135.camel@surriel.com> <856ed0da-d806-4f2e-95e7-f276f2408e8d@kernel.org> <4ffa2c82c538c12447d48fee2532f67c35f7eab7.camel@surriel.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/3/26 18:00, Rik van Riel wrote: > On Tue, 2026-11-03 at 17:39 +0100, David Hildenbrand (Arm) wrote: >> On 11/3/26 17:29, Rik van Riel wrote: >>> >>> faultin_page_range() and populate_vma_page_range >>> both call the GUP code with a NULL pages pointer. >> >> That's not what I meant. I know that we have GUP users without >> pages array. >> >> In the code change you are performing the check is not required. >> >> That's why there wasn't one in the first place. > > It looks like 2 of the 3 callers of gup_fill_pages() > check for pages existing. The other does not. > > Should we just have the check in this one place, and > not depend on the caller? > That should be spelled out in the patch description. The change as it stands in this patch looks useless :) Although trying to fill something that doesn't exist is a bit weird. But if it makes the calling code nicer, sure. >> >> >>> >>> However, now we have a situation where: >>> 1) Only a few oddball callers of GUP need the dcache flush, and >>> 2) Those need to do their OWN flush after a write, and >>> 3) The dcache flush is not needed for most other GUP callers. >>> >>> We could do two different things here to solve the >>> asymmetry. >>> >>> Either we could add dcache flushing to  >>> unpin_user_pages_dirty_lock(), and make sure all >>> the callers that need to call that function, or >>> we could move the cache flushing to the GUP callers. >>> >>> Then we have KSM, which uses GUP, and which would like >>> to see the latest page contents... >> >> It's all very inconsistent. And undocumented. And error prone. >> >> Could we have GUP-fast also flush the dcache for consistency or >> must that only be called with the PT lock held? > > I think we could, but it does not look like any of > them need it. Sure, but it makes the code more consistent and from a GUP PoV easier to understand. > > Out of the 83 callers of gup_fast, many DMA into the > page, and the drivers do their own flushing. Also, > flushing before the DMA write doesn't help. > > KVM, Infiniband, and others pin the pages, sometimes > for longer periods of time, and either flush around  > their reads and writes, or run only on architectures > where we don't need to. > > The others access the page through the user's own > virtual address, which again does not require a > dcache flush. > > Only a small number of slow GUP callers need dcache > flushes. > > It may make more sense to move the cache flushes > all the way out into __get_user_pages() and away > from the follow_page_mask() call chain, so we can > maybe merge more of the helper functions between > slow GUP and fast GUP. That's likely more work, can you look into that? -- Cheers, David