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 9A1B8325729; Wed, 26 Nov 2025 12:19:07 +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=1764159548; cv=none; b=Zgm2RqqqxIJjgOaJ0UL1QnrupCocJr56DyiklUxizuYpKstuY1n0PJrHS5+/sykK3ie/jHBzs1f0bf+ff5hdF+ObKj1OBXC4O63S+OMbvFt780P0n8m8CV8h9ulw6fegCzkXBBvJ4KAl/nJlmejf9txkdZoyGJsP4x8xBS26/2o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764159548; c=relaxed/simple; bh=ma7x77mcp++yOaA+tQf5VwWQgC8A62xYMabV/ICqieo=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=ZhMUDbgh+XLCWKgimJRnbNPb9KA/Ek3/scZgQLPTtsN7060GqiCf1qaQAmUolxPHb6ta+/DB0cZ6heODLK6UFhccf2wlVNKs7FmZ/h46AZ0fk1y2TbnFJn2NNijsP1fRzGaOcstEajzDCB+JXo00FqDi7htxVuX8A81DLdKpBto= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=D3f6skmv; 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="D3f6skmv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A7348C113D0; Wed, 26 Nov 2025 12:19:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764159547; bh=ma7x77mcp++yOaA+tQf5VwWQgC8A62xYMabV/ICqieo=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=D3f6skmvyEwFQ6GcsmWCUMpDzR29kqNoSz21S697+cgAnHCPtqAVpRK5FO8n0foRk w+p7odI+hZYN1BUGcWGQV8rSriDIOZcp+2yPlOIQLhxb+ZRzKQLvLS9FtvhfrVLYZe BCdgexPlLZxAjWXXkGVxlHnxobkWy1eCXPed4keEvMN3sB7CUnBt+VuRfkNrS4RU60 clLCupwtRNlxZy1OKldXhxUwjyJawrzwni1O+v6XadG6I3G3HoqR9INyVC3ziWPWc/ UWP35qM1Ub+uCKi0VmFEur1VMa3LH4V9IgB3w1XS+sJeJylKus4fIhzYikGMuB8Ibe JxOtv+pp0YMcA== Message-ID: Date: Wed, 26 Nov 2025 13:19:00 +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 From: "David Hildenbrand (Red Hat)" 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> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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. -- Cheers David