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X-CSE-ConnectionGUID: 0g0Zzyp7T+C6LcMmO+mPdQ== X-CSE-MsgGUID: xyS/KVf8ROSZtsYM2oUfTQ== X-IronPort-AV: E=McAfee;i="6700,10204,11365"; a="42495392" X-IronPort-AV: E=Sophos;i="6.14,226,1736841600"; d="scan'208";a="42495392" Received: from orviesa009.jf.intel.com ([10.64.159.149]) by orvoesa110.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Mar 2025 08:09:13 -0800 X-CSE-ConnectionGUID: aveQA0pmQyyg28REEV5zCA== X-CSE-MsgGUID: tOd5QqqARrqslTXHXpQOmQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.14,226,1736841600"; d="scan'208";a="118798282" Received: from vverma7-desk1.amr.corp.intel.com (HELO [10.125.110.31]) ([10.125.110.31]) by orviesa009-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Mar 2025 08:09:11 -0800 Message-ID: <716f5a42-6828-4f0e-9002-d3bd5c4ad363@intel.com> Date: Thu, 6 Mar 2025 08:09:33 -0800 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 1/1] x86/vmemmap: Use direct-mapped VA instead of vmemmap-based VA To: Andrew Morton , Gwan-gyeong Mun Cc: "Harry Yoo (Oracle)" <42.hyeyoo@gmail.com>, linux-kernel@vger.kernel.org, osalvador@suse.de, byungchul@sk.com, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, max.byungchul.park@sk.com, max.byungchul.park@gmail.com, x86@kernel.org References: <20250217114133.400063-1-gwan-gyeong.mun@intel.com> <20250217114133.400063-2-gwan-gyeong.mun@intel.com> <20250217214342.33b4fcc4e775cf5ca70053a0@linux-foundation.org> <06eff380-a386-44d1-9115-33a08c2d8c95@intel.com> <20250305194641.357826b606a53f98474eb997@linux-foundation.org> Content-Language: en-US From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/5/25 19:46, Andrew Morton wrote: > Can we please have review from x86 maintainers? I didn't respond here because I thought this was the same problem from the same contributor that we addressed in another thread: > https://lore.kernel.org/all/d1da214c-53d3-45ac-a8b6-51821c5416e4@intel.com/ I think this approach is a hack. It basically requires that every bit of code that _might_ update (and then use) a PGD in the init_mm know how to find the direct map alias and then use that instead. This would further specialize the x86 code. I have no reason to believe that this is truly an x86-specific problem. Are we really the only arch that has a per-process PGD that maps the shared kernel page tables? The right solution (like I mentioned in the other thread) is to sync the PGDs more aggressively. Syncing them is expensive, of course, but it's a pretty darn rare operation.