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 B84DD328622; Wed, 26 Nov 2025 12:35:24 +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=1764160524; cv=none; b=P8WRMHzSMujNtWnyRZJRApiSmSg7MAE5e1khiR9quQ+ASTJUyX/MeIskFs4FCkgu8nRmnTh4aDvi22oHk4ciPpqoT0uOd0erdc+10CN6C6u3Hp+tPM28MC/r/aT0l5B3YpxL2zdjSXh/FJv7M++/uZIfVsMuD2zOl5UsN/QVGCE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764160524; c=relaxed/simple; bh=rXwhATyOSB0m9x5O2nBFhUNTR3ETov+bBvaSAYz2hFU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OKiAHpunDvD8A44+CZVQlyO6K7r4iZ9aIJepRMROdmas541Oazjl+ObFV7Xy2+fPEAM/H8dM6GZo6Od6bH9/9qFB3gnL8EfbFAmdqwyIMUVq4HTV5HsyfAQgEzMX1H+3rvNTdt8jzo8MqJZdBgMAxY90dlKOV1TUTJUqHFKmEkk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ueGpeTww; 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="ueGpeTww" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C8C6FC113D0; Wed, 26 Nov 2025 12:35:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764160524; bh=rXwhATyOSB0m9x5O2nBFhUNTR3ETov+bBvaSAYz2hFU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ueGpeTww704+deIDOHRU6afye/CjFOPnI68YfLCYGwa2NIBPqhUBIG9JbhbzRNOlQ MDosOn8WFxDBLdQjMUPJzxabKUIYqrxykB8/Ijg5DJ97xWCgkyH+V3/uZfH2HEiV48 2K+AvWiwMmOZWZgAuUZU1DF7SHQ6rB99llGyPyoUHqmCcwWWeXDZuerABOAfmckBoB Fz49pQs3cVvv03oyAhYOK/uOX3N7UpWpjPxK1Rru+yGugebJwEwZR5a0azN71kSdh8 Ygm00azLCFgyKYsfXl3GthpfDfeTd+HRtirSPED6tqpjyHwnLb5PXOKwpEB/boWyFp eWdeSrdF4KsLw== Message-ID: <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> Date: Wed, 26 Nov 2025 13:35:17 +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: Lorenzo Stoakes Cc: Ryan Roberts , Samuel Holland , Palmer Dabbelt , Paul Walmsley , linux-riscv@lists.infradead.org, Andrew Morton , linux-mm@kvack.org, devicetree@vger.kernel.org, Suren Baghdasaryan , linux-kernel@vger.kernel.org, Mike Rapoport , Michal Hocko , Conor Dooley , 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> <6bdf2b89-7768-4b90-b5e7-ff174196ea7b@lucifer.local> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <6bdf2b89-7768-4b90-b5e7-ff174196ea7b@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/26/25 13:27, Lorenzo Stoakes wrote: > On Wed, Nov 26, 2025 at 01:19:00PM +0100, David Hildenbrand (Red Hat) wrote: >> On 11/26/25 13:16, David Hildenbrand (Red Hat) wrote: >>> 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? >>> >> >> Just stumbled over the reply from Christope: >> >> https://lkml.kernel.org/r/0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org >> >> And wonder if we could handle that somehow directly in the pgdp_get() etc. > > I find that kind of gross to be honest. Isn't the whole point of folding that we > don't have to think about it... If we could adjust generic pgdp_get() and friends to not do a READ_ONCE() once folded we might not have to think about that in the callers. Just an idea, though, not sure if that would fly the way I envision it. -- Cheers David