From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 8A19B3E171C for ; Wed, 3 Jun 2026 15:49:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780501743; cv=none; b=fgQV2sSN007YiqkGjuJpr8tbnSNbu3s50cg/cXT9ZRAprEBmLqpfDmAhaHCMGPC00HvcDPxG1Fw8Vh0Ep7W8ACVhjRwoVWQlGE2op8yxHlt/fs854YRUdduvYjqk+nhUB/8Xrz7hO8Nuaenb1kOAEEs7U4Mh9nvS19m0hZw3X4Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780501743; c=relaxed/simple; bh=BDeHK5wFPxXzuHdqbENLApehuK/WF3qoZ9VCBG55MxI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=m92OOAIzIPmuBJh8j12hGpitOGNn0SbEekpVYiDgM7QZwK7upCA0Q+7UOckK4I/4zyu0UZPzV2jN1T+gqkyYVaqLAJMyj9uQLbUWh+3sGvoZJmxHZM9kACtjVwd93KDHm8vYRBIVIS+OsggSGALi3qsOrvizwNfa2z/m53IJZRM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=U6pbP/IJ; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="U6pbP/IJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5E4881F00893; Wed, 3 Jun 2026 15:48:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780501742; bh=sYUk6vW20DB8I7Vftloyl0dbtzWNes19X5Rj0mk5KyQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=U6pbP/IJZ5p1e2csLq0FLEBF3slLLyoJBCa/7Gw3ja4mHFR7RrPzQqJQ1mwDpj9r5 FTzlfUp4aySgreHzJ/77ZhBvpYaBwo9PHPy5+dRrdYidEnal4iyRNiq5uWH0ZGJVFq ydnzXgYp97Tcute5q/pHMrMQR60ydBZYfxHEZKu4tAJ6Gp10+1kisPqXQ4+X3Aqw5j XfzlWU9uDsQG3fHS2em15jeQFvi5Gqck086IZXBv2De8q+PQlNyYws17YzpNY69AzO DgXQrJmMtShNbkmmRdj0E1wWLMshIeHHwE3X3I0FbauymGr84g9NfAARyM/dWRlMed txBsRqa347D3Q== Message-ID: Date: Wed, 3 Jun 2026 17:48:56 +0200 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: [RFC PATCH 1/2] mm/secretmem: try to restore large page mappings in direct map To: Lance Yang , Mike Rapoport Cc: akpm@linux-foundation.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, luto@kernel.org, peterz@infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, xueyuan.chen21@gmail.com, ioworker0@gmail.com References: <20260603114134.3010-1-lance.yang@linux.dev> <3ec76ffd-2650-4302-87e1-8183029a0553@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/3/26 15:09, Lance Yang wrote: > > > On 2026/6/3 20:35, Mike Rapoport wrote: >> On Wed, Jun 03, 2026 at 07:41:34PM +0800, Lance Yang wrote: >>> >>> Good point, I kept it separate on purpose :) >>> >>> Putting collapse into set_direct_map_default_noflush() would change the >>> semantics of that helper a bit, IMHO. >> >> For x86 default means present + rw + PSE, so yuu can look at it as actually >> better enforcing the semantics :) > > Yep. One x86 detail though, default seems to miss _PAGE_GLOBAL today. Not > sure if that is intentional or just historical. See patch #02. > >>> I would expect arch_try_collapse_direct_map() to also be useful for cases >>> where a direct-map permission change could split a large maping first, >>> and the user wants to try restoring the large mapping after changing it >>> back. One example[1] is making a direct-map range read-only for security, >>> which I am also working on :) >> >> I don't think users should care. The users care for particular permissions >> of a range in the direct map. It should be up to the architecture to select >> most suitable mapping size. The splits are implicit, I don't see why >> collapses can't be implicit as well. > > And agreed, users should not care about the final mapping size, that is > up to the arch. > > TBH, my concern is making the collapse cost implicit for every > set_direct_map_default_noflush() caller. I still lean toward keeping > it opt-in, but happy to hear what folks prefer :) If we could easily do that automatically, that would likely be preferable. Especially given that we are getting other users of direct-map removal soon that would face similar problems (e.g., guest_memfd). What the performance impact of trying to collapse after every directmap update? Imagine we have a full PMD range with directmap-removed PTEs? -- Cheers, David