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 1D2AA24B248; Fri, 10 Jan 2025 19:18:34 +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=1736536715; cv=none; b=CtK0z6iKeVRdQVCFrA7wQeWIEphwWEFLAbkSTvTEwBPqqmwukLQjHI3lYudgrVWvquAqjc9Eu8yjPPJnPlkAUqroKj9N5l759VZT29uq2ie7cgR0eupNsqeH7R21a1EmpmUwNPRm3LxqYkSYSt3ZHszNxhIMT7ewFGChCzGJJG8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1736536715; c=relaxed/simple; bh=2qtn/yN8KGnvKM4Be6ymIMRepfChNk260BsIzmavFNU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=BcOx9WM7MED90MDfxMUQ/dAwQs8WXGn23K5YOIhd3gQPtA3x1TlfMPguVxVqdgdc9GF2d//U2dbZI+YrTb6Wqk8cYxFTico3veMUares8+ZcZMMDz2GoHv0GWopwOirMNijPAAjxhX837+s4OpU8HtgqKG5A5LZ9wegR3WNb9AE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=lVSbNBfB; 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="lVSbNBfB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9D2BDC4CED6; Fri, 10 Jan 2025 19:18:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1736536714; bh=2qtn/yN8KGnvKM4Be6ymIMRepfChNk260BsIzmavFNU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=lVSbNBfBgiPxfs3j9J4+Q/Mm+va0HnyJJvJ511BIZPHoPIheFgi517TCooWsbBMUd WSTmKdJWe1UfTSlATsW95ACeJ+2Ll0h4BtuuaSFwVIFk0Wv0zZu3LZGeJRFeQJn0qe lg2p9xQs3O5V2HOaVWmRlOJx02Yzw7s7vpvz4fokLmZ7ey61g0GQZuRvribH6nHyqr xDH3Wpe0L5t74rdpA5UINqI4Hxxsp6lB+b5KVRB/rten4cAybUcKdl/88Oc2pCe2TE 6b7kfjQEJbS5HJyWvsAtyyHoKwfXfP3N4KXgVgUwkJm645MDTdxDa3wPAeEVyf6U26 meOgl0uiogQDw== Date: Fri, 10 Jan 2025 11:18:32 -0800 From: Luis Chamberlain To: "Kirill A. Shutemov" Cc: Mike Rapoport , Andrew Morton , Andy Lutomirski , Anton Ivanov , Borislav Petkov , Brendan Higgins , Daniel Gomez , Daniel Thompson , Dave Hansen , David Gow , Douglas Anderson , Ingo Molnar , Jason Wessel , Jiri Kosina , Joe Lawrence , Johannes Berg , Josh Poimboeuf , "Kirill A. Shutemov" , Mark Rutland , Masami Hiramatsu , Miroslav Benes , "H. Peter Anvin" , Peter Zijlstra , Petr Mladek , Petr Pavlu , Rae Moar , Richard Weinberger , Sami Tolvanen , Shuah Khan , Song Liu , Steven Rostedt , Thomas Gleixner , kgdb-bugreport@lists.sourceforge.net, kunit-dev@googlegroups.com, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-modules@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-um@lists.infradead.org, live-patching@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH 3/8] x86/mm/pat: Restore large pages after fragmentation Message-ID: References: <20241227072825.1288491-1-rppt@kernel.org> <20241227072825.1288491-4-rppt@kernel.org> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Jan 10, 2025 at 12:36:59PM +0200, Kirill A. Shutemov wrote: > On Fri, Dec 27, 2024 at 09:28:20AM +0200, Mike Rapoport wrote: > > From: "Kirill A. Shutemov" > > > > Change of attributes of the pages may lead to fragmentation of direct > > mapping over time and performance degradation as result. > > > > With current code it's one way road: kernel tries to avoid splitting > > large pages, but it doesn't restore them back even if page attributes > > got compatible again. > > > > Any change to the mapping may potentially allow to restore large page. > > > > Hook up into cpa_flush() path to check if there's any pages to be > > recovered in PUD_SIZE range around pages we've just touched. > > > > CPUs don't like[1] to have to have TLB entries of different size for the > > same memory, but looks like it's okay as long as these entries have > > matching attributes[2]. Therefore it's critical to flush TLB before any > > following changes to the mapping. > > > > Note that we already allow for multiple TLB entries of different sizes > > for the same memory now in split_large_page() path. It's not a new > > situation. > > > > set_memory_4k() provides a way to use 4k pages on purpose. Kernel must > > not remap such pages as large. Re-use one of software PTE bits to > > indicate such pages. > > > > [1] See Erratum 383 of AMD Family 10h Processors > > [2] https://lore.kernel.org/linux-mm/1da1b025-cabc-6f04-bde5-e50830d1ecf0@amd.com/ > > > > [rppt@kernel.org: > > * s/restore/collapse/ > > * update formatting per peterz > > * use 'struct ptdesc' instead of 'struct page' for list of page tables to > > be freed > > * try to collapse PMD first and if it succeeds move on to PUD as peterz > > suggested > > * flush TLB twice: for changes done in the original CPA call and after > > collapsing of large pages > > ] > > > > Link: https://lore.kernel.org/all/20200416213229.19174-1-kirill.shutemov@linux.intel.com > > Signed-off-by: Kirill A. Shutemov > > Co-developed-by: Mike Rapoport (Microsoft) > > Signed-off-by: Mike Rapoport (Microsoft) > > When I originally attempted this, the patch was dropped because of > performance regressions. Was it addressed somehow? > Also, the statement: "Change of attributes of the pages may lead to fragmentation of direct mapping over time and performance degradation as result. " Seems to contradict the findings reported at LSFMM by Mike before that direct map fragmentation does not incur performance penalty, so I don't see a point in contradicing those findings and confusing people further. Luis