From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id EB6CB1B4223; Wed, 26 Nov 2025 13:03:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764162232; cv=none; b=rrPXmNqFX4g4+pq0D7m5wESW5YsBnt6Eahq+KRpqQZ+V7UrxxPRHT2Qrbpxaz5Owu7eja++aH0uNG7FA8THwo3Kgq1PivEpySm/WT9j+jUalMuulaTqAomLemSKqMqv5gAzeFOL835oZGq7L1SICGTTP6BYyKQwhmIFl37r3U9E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764162232; c=relaxed/simple; bh=GJxg+mOomDF6t+bzwy7AaCjkDWkZiNI/cJ3AyC/vl7I=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IgiJFb1kz/RC3aypwyJWWOu4LPwk/SeD/DCsB2m8e5aA39jdzdiJwpJAF9FSWX7mpcbf283G1MgsQ5v1rLsgIuLpbtgywgbqYpykhhFvzoojleTRF0H+f1h3BW63SdhnWGVMtqJ5iJr3OZTdcTIsa7ycP1UOvj67m1M+oa0bXG0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5553A168F; Wed, 26 Nov 2025 05:03:39 -0800 (PST) Received: from [10.1.33.153] (unknown [10.1.33.153]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1FDE83F66E; Wed, 26 Nov 2025 05:03:44 -0800 (PST) Message-ID: Date: Wed, 26 Nov 2025 13:03:42 +0000 Precedence: bulk X-Mailing-List: linux-kernel@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 Content-Language: en-GB To: "David Hildenbrand (Red Hat)" , Lorenzo Stoakes Cc: 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> <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> From: Ryan Roberts In-Reply-To: <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 26/11/2025 12:35, David Hildenbrand (Red Hat) wrote: > 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 certainly don't like the suggestion of doing the is_folded() test outside the helper, but if we can push that logic down into pXdp_get() that would be pretty neat. Anshuman and I did briefly play with the idea of doing a C dereference if the level is folded and a READ_ONCE() otherwise, all inside each pXdp_get() helper. Although we never proved it to be correct. I struggle with the model for folding. Do you want to optimize out all-but-the-highest level's access or all-but-the-lowest level's access? Makes my head hurt... >> >> 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... Agreed, but if we can put it inside the default helper implementation, that solves it, I think? An arch has to be careful if they are overriding the defaults, but it's still well contained. > > 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.