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 CFAF3C54E94 for ; Mon, 23 Jan 2023 16:43:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233378AbjAWQnZ (ORCPT ); Mon, 23 Jan 2023 11:43:25 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47452 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233294AbjAWQnT (ORCPT ); Mon, 23 Jan 2023 11:43:19 -0500 Received: from smtp-out2.suse.de (smtp-out2.suse.de [IPv6:2001:67c:2178:6::1d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 582D72BF38; Mon, 23 Jan 2023 08:42:50 -0800 (PST) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id E330F1F45B; Mon, 23 Jan 2023 16:42:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1674492138; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=A1U4rA+VoFO9h3GdxtZVKjzAASXf334IjkmplB6VlGQ=; b=xc/cvDDxdcoScBSeKZyWHi9Pmc4XEAffKczz1UJGoADpR5KOlL3ERGopUkTej4M3cVOCxf nBrPCP3JSUKYRwOj1XhvgS6Mdr8Tnhi/ba9NtQRWMUD7FyMkZ68F0QFZSVzeVrnJZtTxLe 9dycIdivY1+DgPQsFpZkEKB8QkiU2bY= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1674492138; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=A1U4rA+VoFO9h3GdxtZVKjzAASXf334IjkmplB6VlGQ=; b=wivDXhSuA+RJe2xPtE+LrPo+hQfDN+VNt4pwi9X+3FEFKLq5ap7D5AnLZI//PfxWEtSYIi L8qM4Jr4lm6wAzCQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id D3E53134F5; Mon, 23 Jan 2023 16:42:18 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id ZiS7M+q4zmP+fwAAMHmgww (envelope-from ); Mon, 23 Jan 2023 16:42:18 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 56191A06B5; Mon, 23 Jan 2023 17:42:18 +0100 (CET) Date: Mon, 23 Jan 2023 17:42:18 +0100 From: Jan Kara To: Matthew Wilcox Cc: David Howells , John Hubbard , Al Viro , Christoph Hellwig , Jens Axboe , Jan Kara , Jeff Layton , Logan Gunthorpe , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list) Message-ID: <20230123164218.qaqqg3ggbymtlwjx@quack3> References: <20230120175556.3556978-1-dhowells@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 23-01-23 16:31:32, Matthew Wilcox wrote: > On Fri, Jan 20, 2023 at 05:55:48PM +0000, David Howells wrote: > > (3) Make the bio struct carry a pair of flags to indicate the cleanup > > mode. BIO_NO_PAGE_REF is replaced with BIO_PAGE_REFFED (equivalent to > > FOLL_GET) and BIO_PAGE_PINNED (equivalent to BIO_PAGE_PINNED) is > > added. > > I think there's a simpler solution than all of this. > > As I understand the fundamental problem here, the question is > when to copy a page on fork. We have the optimisation of COW, but > O_DIRECT/RDMA/... breaks it. So all this page pinning is to indicate > to the fork code "You can't do COW to this page". > > Why do we want to track that information on a per-page basis? Wouldn't it > be easier to have a VM_NOCOW flag in vma->vm_flags? Set it the first > time somebody does an O_DIRECT read or RDMA pin. That's it. Pages in > that VMA will now never be COWed, regardless of their refcount/mapcount. > And the whole "did we pin or get this page" problem goes away. Along > with folio->pincount. Well, but anon COW code is not the only (planned) consumer of the pincount. Filesystems also need to know whether a (shared pagecache) page is pinned and can thus be modified behind their backs. And for that VMA tracking isn't really an option. Honza -- Jan Kara SUSE Labs, CR