From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3607426ED5F; Wed, 26 Nov 2025 12:16:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764159403; cv=none; b=WRPOp/nehcWkmpPzuqfkmDgq9pg3ZvJgs9prLdsGqvHbDWcxH/U4aYhbVN5990VcpTlRE5QVEz0c6TYWM13tXQBTivEtLZq9f3heyHw+j88YBl3DhM9mPVyl9FYQrh43bauXD8IU0GbVZmcd6sAvJM68Jmim2DT9fK1X4UKuSyo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764159403; c=relaxed/simple; bh=569JL8Idd3SiwMOD93rNhvlIpUh+BPTaH7EbV9ymg/w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BvvuQnNW/FWQn+Kz2xPR5p+L82/5WCLLJC6iPJtRWkWO8ZMDIDdNxWeclvekV4gFLhnyCCvzkzbJZkOHtSB30XGXCpzdZw2wtncMiP90J+zvtxKxcKUbuOE6QMJeTG9almVmE5KkBHz8JIdLnxXPdf+X9Oubi7d6166YfV5JwJg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TACbEM73; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TACbEM73" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 365DFC113D0; Wed, 26 Nov 2025 12:16:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764159402; bh=569JL8Idd3SiwMOD93rNhvlIpUh+BPTaH7EbV9ymg/w=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=TACbEM73IGgECtM+Jde3dTQ6OgN+Wh3e9eSceZE9/38PANZYd9Dm5t5nHqzypK0NH XTiCzEQCy8XCf+z2hzbZjcFdNTkGDX4ngF1SbU0txbSHPmQp3ZThXVE6Xh6sD6sU52 83TDBRfkmuGwfkNbxf4rvLFAgMgQM8VejwxYHgd6gXyEGnPMIo1fsAskyUH6dr92Nh CYen0EIEPfkFy72coiuPo4TWQSB+ayBrz5FnlYBq2yPe1Z4Bg1TXVk8DfLWPqaI2zx B7dp28/HSQqCOi6qVLdB518TEL6vYLFhMIh6kiB72HzuAeAefTha1GwkvD7PlB3KlW vDM+ci1MJj66g== Message-ID: Date: Wed, 26 Nov 2025 13:16:35 +0100 Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 06/22] mm: Always use page table accessor functions To: Ryan Roberts , Samuel Holland , Palmer Dabbelt , Paul Walmsley , linux-riscv@lists.infradead.org, Andrew Morton , linux-mm@kvack.org Cc: devicetree@vger.kernel.org, Suren Baghdasaryan , linux-kernel@vger.kernel.org, Mike Rapoport , Michal Hocko , Conor Dooley , Lorenzo Stoakes , Krzysztof Kozlowski , Alexandre Ghiti , Emil Renner Berthing , Rob Herring , Vlastimil Babka , "Liam R . Howlett" , Julia Lawall , Nicolas Palix , Anshuman Khandual References: <20251113014656.2605447-1-samuel.holland@sifive.com> <20251113014656.2605447-7-samuel.holland@sifive.com> <02e3b3bd-ae6a-4db4-b4a1-8cbc1bc0a1c8@arm.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <02e3b3bd-ae6a-4db4-b4a1-8cbc1bc0a1c8@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/26/25 12:09, Ryan Roberts wrote: > On 13/11/2025 01:45, Samuel Holland wrote: >> Some platforms need to fix up the values when reading or writing page >> tables. Because of this, the accessors must always be used; it is not >> valid to simply dereference a pXX_t pointer. >> >> Fix all of the instances of this pattern in generic code, mostly by >> applying the below coccinelle semantic patch, repeated for each page >> table level. Some additional fixes were applied manually, mostly to >> macros where type information is unavailable. >> >> In a few places, a `pte_t *` or `pmd_t *` is actually a pointer to a PTE >> or PMDE value stored on the stack, not a pointer to a page table. In >> those cases, it is not appropriate to use the accessors, because the >> value is not globally visible, and any transformation from pXXp_get() >> has already been applied. Those places are marked by naming the pointer >> `ptentp` or `pmdvalp`, as opposed to `ptep` or `pmdp`. >> >> @@ >> pte_t *P; >> expression E; >> expression I; >> @@ >> - P[I] = E >> + set_pte(P + I, E) >> >> @@ >> pte_t *P; >> expression E; >> @@ >> ( >> - WRITE_ONCE(*P, E) >> + set_pte(P, E) >> | >> - *P = E >> + set_pte(P, E) >> ) > > There should absolutely never be any instances of core code directly setting an > entry at any level. This *must* always go via the arch code helpers. Did you > find any instances of this? If so, I would consider these bugs and suggest > sending as a separate bugfix patch. Bad things could happen on arm64 because we > may need to break a contiguous mapping, which would not happen if the value is > set directly. > >> >> @@ >> pte_t *P; >> expression I; >> @@ >> ( >> &P[I] >> | >> - READ_ONCE(P[I]) >> + ptep_get(P + I) >> | >> - P[I] >> + ptep_get(P + I) >> ) >> >> @@ >> pte_t *P; >> @@ >> ( >> - READ_ONCE(*P) >> + ptep_get(P) >> | >> - *P >> + ptep_get(P) >> ) > > For reading the *PTE*, conversion over to ptep_get() should have already been > done (I did this a few years back when implementing support for arm64 contiguous > mappings). If you find any cases where direct dereference or READ_ONCE() is > being done in generic code for PTE, then that's a bug and should also be sent as > a separate patch. > > FYI, my experience was that Coccinelle didn't find everything when I was > converting to ptep_get() - although it could have been that my Cochinelle skills > were not up to scratch! I ended up using an additional method where I did a > find/replace to convert "pte_t *" to "ptep_handle_t" and declared pte_handle_t > as a void* which causes a compiler error on dereference. Then in a few key > places I did a manual case from pte_handle_t to (pte_t *) and compiled allyesconfig. > > I'm assuming the above Cocchinelle template was also used for pmd_t, pud_t, > p4d_t and pgd_t? > >> >> Additionally, the following semantic patch was used to convert PMD and >> PUD references inside struct vm_fault: >> >> @@ >> struct vm_fault vmf; >> @@ >> ( >> - *vmf.pmd >> + pmdp_get(vmf.pmd) >> | >> - *vmf.pud >> + pudp_get(vmf.pud) >> ) >> >> @@ >> struct vm_fault *vmf; >> @@ >> ( >> - *vmf->pmd >> + pmdp_get(vmf->pmd) >> | >> - *vmf->pud >> + pudp_get(vmf->pud) >> ) >> >> >> Signed-off-by: Samuel Holland >> --- >> This commit covers some of the same changes as an existing series from >> Anshuman Khandual[1]. Unlike that series, this commit is a purely >> mechanical conversion to demonstrate the RISC-V changes, so it does not >> insert local variables to avoid redundant calls to the accessors. A >> manual conversion like in that series could improve performance. >> >> [1]: https://lore.kernel.org/linux-mm/20240917073117.1531207-1-anshuman.khandual@arm.com/ > > Hi, > > I've just come across this patch and wanted to mention that we could also > benefit from this improved absraction for some features we are looking at for > arm64. As you mention, Anshuman had a go but hit some roadblocks. > > The main issue is that the compiler was unable to optimize away the READ_ONCE()s > for the case where certain levels of the pgtable are folded. But it can optimize > the plain C dereferences. There were complaints the the generated code for arm > (32) and powerpc was significantly impacted due to having many more (redundant) > loads. > We do have mm_pmd_folded()/p4d_folded() etc, could that help to sort this out internally? -- Cheers David