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Fri, 18 Sep 2026 07:04:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 136851F000FF; Fri, 18 Sep 2026 07:03:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789715043; bh=C99qchwVmIf8eGzA2KZRAlNCqTMDn2E/1eNOjto5V1g=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=jWKDEaWgQ8tOV8amlp5t9MgYwzBijBFVifWz9yuQo8X4L5EFh32Flzu4XXAbDfnGm UDL93p7kB9G3Rd42nYwUtj6uT0m3roRBv2y8vs/mC/hhbavAalLWNzIamZ9CawgJa5 pHnOvDOLIlc6CP9lOwT216XgLvfodA7hkiX+FB/0Ct+bfDmyj/AqncmimtUagrt8Eb oF0sbTAHHKTDObi0AO6aHDc2FHQ2Fy45mE5vliTDkO5ZU6YC6Yt14abrG5j+Mj0rNr Yjv8AMXd9/oiS5ceTffHmQV+p3xP+CkZLZuUIpC2CQ619hXKr9pxjr41fijo1ig8F1 DgCMlNLI1L+8Q== Message-ID: <24aedc8c-a5c6-418b-a37e-051f0d6d9fa7@kernel.org> Date: Fri, 18 Sep 2026 09:03:55 +0200 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 03/10] mm/vmalloc: use pte_set_huge()/pte_clear_huge() for PTE-level block mappings To: Barry Song Cc: Wen Jiang , akpm@linux-foundation.org, catalin.marinas@arm.com, linux-mm@kvack.org, urezki@gmail.com, will@kernel.org, Xueyuan.chen21@gmail.com, ajd@linux.ibm.com, anshuman.khandual@arm.com, david@kernel.org, dev.jain@arm.com, jiangwen6@xiaomi.com, leo.yan@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, rppt@kernel.org, ryan.roberts@arm.com References: <20260917052933.188679-1-jiangwenxiaomi@gmail.com> <20260917052933.188679-4-jiangwenxiaomi@gmail.com> <27072d94-13bf-4bea-948a-d8a67976601e@kernel.org> <2462f67c-f7d3-4eb3-a093-6a338ef49f14@kernel.org> Content-Language: fr-FR From: "Christophe Leroy (CS GROUP)" In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Le 18/09/2026 à 08:14, Barry Song a écrit : > On Fri, Sep 18, 2026 at 2:05 PM Christophe Leroy (CS GROUP) > wrote: >> >> >> >> Le 17/09/2026 à 23:44, Barry Song a écrit : >>> On Thu, Sep 17, 2026 at 10:41 PM Wen Jiang wrote: > [...] > >>>>>> >>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>>>>> index cdd68ed3ae1a9..349ced999f959 100644 >>>>>> --- a/include/linux/pgtable.h >>>>>> +++ b/include/linux/pgtable.h >>>>>> @@ -2134,6 +2134,35 @@ static inline int pmd_free_pte_page(pmd_t *pmd, unsigned long addr) >>>>>> } >>>>>> #endif /* CONFIG_HAVE_ARCH_HUGE_VMAP */ >>>>>> >>>>>> +/* >>>>>> + * PTE-level block mappings for vmap. >>>>>> + * >>>>>> + * pte_set_huge() only has to be implemented by architectures whose >>>>>> + * arch_vmap_pte_range_map_size() can return a size other than PAGE_SIZE. >>>>>> + */ >>>>>> +#ifndef __HAVE_ARCH_PTE_SET_HUGE >>>>>> +static inline void pte_set_huge(pte_t *ptep, unsigned long addr, >>>>>> + phys_addr_t phys, pgprot_t prot, >>>>>> + unsigned long size) >>>>>> +{ >>>>>> + WARN_ON_ONCE(1); >>>>> >>>>> BUILD_BUG_ON() would be better here. >>>>> >>>>> It should be possible because fallback arch_vmap_pte_range_map_size() >>>>> will constant-fold PAGE_SIZE so pte_set_huge() will never be called. >>>>> >>>> >>>> Agreed. These fallbacks exist only to keep the build working on >>>> architectures with PTE-level block mappings and should never actually >>>> be reached, so BUILD_BUG_ON() is right. I'll make that change in v9. >>>> >>> >>> I am not quite sure. It won't be called at runtime because >>> `vmap size`/`unmap size` return `PAGE_SIZE`, so the code won't >>> reach this branch. But it will still be built. >> >> The fallbacks are defined as: >> >> #ifndef arch_vmap_pte_range_map_size >> static inline unsigned long arch_vmap_pte_range_map_size(unsigned long >> addr, unsigned long end, >> u64 pfn, unsigned int max_page_shift) >> { >> return PAGE_SIZE; >> } >> #endif >> >> #ifndef arch_vmap_pte_range_unmap_size >> static inline unsigned long arch_vmap_pte_range_unmap_size(unsigned long >> addr, >> pte_t *ptep) >> { >> return PAGE_SIZE; >> } >> #endif >> >> Therefore in: >> >> size = arch_vmap_pte_range_unmap_size(addr, pte); >> if (size != PAGE_SIZE) { >> >> GCC knows 'size' is const and its value is PAGE_SIZE, so it won't emit >> the branch at all. >> >> It is call constant folding, some explanation here: >> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FConstant_folding&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C869dac21f28d45988a3d08df154c225a%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639253088878897551%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=Dvtz1%2FAHvAbVYO%2BRaHTOizcD%2FclJ9FqTcWXKMhIFVyw%3D&reserved=0 >> >>> >>> So, would a `BUILD_BUG_ON()` trigger a build failure here? >> >> It shouldn't, if it does it is a compiled bug or this is because someone >> has redefined arch_vmap_pte_range_map_size() and not pte_set_huge() >> which we'd better know at build time rather than at runtime. > > Thanks, Christophe. I was also thinking about compiler optimization. I > was just a bit worried that we're touching the common MM code, which > affects almost all architectures, so I'm not quite sure whether this is > supported by all GCC versions used by those architectures. > > If it is supported by all of them, I agree that `BUILD_BUG_ON()` is a > perfect approach. AFAIU this is the assumption made by the kernel, see https://docs.kernel.org/process/coding-style.html#conditional-compilation This is the same compiler, I see no reason why ability to constant-fold would be dependant on architecture. Christophe