From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E7C33C433F5 for ; Tue, 8 Mar 2022 17:27:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348285AbiCHR2i (ORCPT ); Tue, 8 Mar 2022 12:28:38 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41864 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349154AbiCHR2g (ORCPT ); Tue, 8 Mar 2022 12:28:36 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 22FF2F7B; Tue, 8 Mar 2022 09:27:36 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id DCA1B61306; Tue, 8 Mar 2022 17:27:35 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 45E63C340EB; Tue, 8 Mar 2022 17:27:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1646760455; bh=Wyo7DT2LJqEn+VENd6dCx837uI7HnqAB2NMm453HARc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=dM3LX4E3Qz2mCJs1OHF05HvFyJsXPwfK12g/YyBr9000rhGAq4e5fs4GHZ+vYJO2I iEwMLBkTqGImqGhmTWvmELXgLU+flvjC5qVaCuNSk/SK+MH5cCDLcdW0RrKLIydWEq Ls62QOa+hahmahJmMj7KACEMhGwO914/OtLqbj4y5OJzW4Ca5QgUBpvGjTerxD7siH pm3Dw4DPbIHgD1T2kHs+MopokIKVssFgGej0bi8qrRsidue/qdjNoCprDzjFt6oBlW dMgPF+Wa2U1E0QNV4/m43xQ1xonN/sTGRRy7u8wtDI3F3QXmt4WSCEgws4AztjIefd 9Qca7ysLr1XOQ== Date: Tue, 8 Mar 2022 09:27:34 -0800 From: "Darrick J. Wong" To: Christoph Hellwig Cc: Hugh Dickins , Andrew Morton , Mikulas Patocka , Zdenek Kabelac , Lukas Czerner , Miklos Szeredi , Borislav Petkov , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH mmotm v2] tmpfs: do not allocate pages on read Message-ID: <20220308172734.GC1479066@magnolia> References: <20220306092709.GA22883@lst.de> <90bc5e69-9984-b5fa-a685-be55f2b64b@google.com> <20220307064434.GA31680@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220307064434.GA31680@lst.de> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Mar 07, 2022 at 07:44:34AM +0100, Christoph Hellwig wrote: > On Sun, Mar 06, 2022 at 02:59:05PM -0800, Hugh Dickins wrote: > > Mikulas asked in > > https://lore.kernel.org/linux-mm/alpine.LRH.2.02.2007210510230.6959@file01.intranet.prod.int.rdu2.redhat.com/ > > Do we still need a0ee5ec520ed ("tmpfs: allocate on read when stacked")? > > > > Lukas noticed this unusual behavior of loop device backed by tmpfs in > > https://lore.kernel.org/linux-mm/20211126075100.gd64odg2bcptiqeb@work/ > > > > Normally, shmem_file_read_iter() copies the ZERO_PAGE when reading holes; > > but if it looks like it might be a read for "a stacking filesystem", it > > allocates actual pages to the page cache, and even marks them as dirty. > > And reads from the loop device do satisfy the test that is used. > > > > This oddity was added for an old version of unionfs, to help to limit > > its usage to the limited size of the tmpfs mount involved; but about > > the same time as the tmpfs mod went in (2.6.25), unionfs was reworked > > to proceed differently; and the mod kept just in case others needed it. > > > > Do we still need it? I cannot answer with more certainty than "Probably > > not". It's nasty enough that we really should try to delete it; but if > > a regression is reported somewhere, then we might have to revert later. > > > > It's not quite as simple as just removing the test (as Mikulas did): > > xfstests generic/013 hung because splice from tmpfs failed on page not > > up-to-date and page mapping unset. That can be fixed just by marking > > the ZERO_PAGE as Uptodate, which of course it is: do so in > > pagecache_init() - it might be useful to others than tmpfs. > > > > My intention, though, was to stop using the ZERO_PAGE here altogether: > > surely iov_iter_zero() is better for this case? Sadly not: it relies > > on clear_user(), and the x86 clear_user() is slower than its copy_user(): > > https://lore.kernel.org/lkml/2f5ca5e4-e250-a41c-11fb-a7f4ebc7e1c9@google.com/ > > > > But while we are still using the ZERO_PAGE, let's stop dirtying its > > struct page cacheline with unnecessary get_page() and put_page(). > > > > Reported-by: Mikulas Patocka > > Reported-by: Lukas Czerner > > Signed-off-by: Hugh Dickins > > I would have split the uptodate setting of ZERO_PAGE into a separate, > clearly documented patch, but otherwise this looks good: > > Reviewed-by: Christoph Hellwig I've long wondered (for my own nefarious purposes) why tmpfs files didn't just grab the zero page, so: Acked-by: Darrick J. Wong --D