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 5311DECAAD8 for ; Fri, 23 Sep 2022 08:44:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230207AbiIWIos (ORCPT ); Fri, 23 Sep 2022 04:44:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56326 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229706AbiIWIoq (ORCPT ); Fri, 23 Sep 2022 04:44:46 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:3::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DA65347B87; Fri, 23 Sep 2022 01:44:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=nNv86JWq7K4SrXi9kThW3IL/MbxUCnwp29q6ItT7Jcw=; b=mZ8CmQuWblk1MMVUpDuLR6qc2s 66kAf6IwGc9rz0v2QxusGWOBx/z7JWCpwJXwtQQfoZl02tWSFIeMLPbv6Y8FLFUcMX5RNQyuMyjNm 2MGSOlBfZUOF3LiHToMr2YMxtlRl+e7cmeIS9x7sZKYj9mQbVlaIHExj1LBNB8vuyeJOWksc5BLvN nPucVivNwS7UH0z21KZ4o09dDV43l7Y1TIlsWlLIFZqIkl+lF4tXhcr6mMihC0eK0a4ng9TjYi2fy Re5oLmrnTKJy1TB7kLdq4LFaKHsUAY6WZuzCKzI5722zzZ/Qq6ez4BCJv73S/3i7ZgPbsi1+l4f4v /WaWCnfw==; Received: from hch by bombadil.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1obeIT-00327F-D2; Fri, 23 Sep 2022 08:44:33 +0000 Date: Fri, 23 Sep 2022 01:44:33 -0700 From: Christoph Hellwig To: Al Viro Cc: Christoph Hellwig , Jan Kara , John Hubbard , Andrew Morton , Jens Axboe , Miklos Szeredi , "Darrick J . Wong" , Trond Myklebust , Anna Schumaker , David Hildenbrand , Logan Gunthorpe , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, LKML Subject: Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin_pages*() routines Message-ID: References: <20220906102106.q23ovgyjyrsnbhkp@quack3> <20220914145233.cyeljaku4egeu4x2@quack3> <20220915081625.6a72nza6yq4l5etp@quack3> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Sep 23, 2022 at 05:22:17AM +0100, Al Viro wrote: > > Add a iov_iter_unpin_pages that does the right thing based on the > > type. (block will need a modified copy of it as it doesn't keep > > the pages array around, but logic will be the same). > > Huh? You want to keep the type (+ direction) of iov_iter in any structure > a page reference coming from iov_iter_get_pages might end up in? IDGI... Why would I? We generall do have or should have the iov_iter around. And for the common case where we don't (bios) we can carry that information in the bio as it needs a special unmap helper anyway. But if you don't want to use the iov_iter for some reason, we'll just need to condense the information to a flags variable and then pass that. > > BTW, speaking of lifetime rules - am I right assuming that fd_execute_rw() > does IO on pages of the scatterlist passed to it? Yes. > Where are they getting > dropped and what guarantees that IO is complete by that point? The exact place depens on the exact taaraget frontend of which we have a few. But it happens from the end_io callback that is triggered through a call to target_complete_cmd. > The reason I'm asking is that here you have an ITER_BVEC possibly fed to > __blkdev_direct_IO_async(), with its > if (iov_iter_is_bvec(iter)) { > /* > * Users don't rely on the iterator being in any particular > * state for async I/O returning -EIOCBQUEUED, hence we can > * avoid expensive iov_iter_advance(). Bypass > * bio_iov_iter_get_pages() and set the bvec directly. > */ > bio_iov_bvec_set(bio, iter); > which does *not* grab the page referneces. Sure, bio_release_pages() knows > to leave those alone and doesn't drop anything. However, what is the > mechanism preventing the pages getting freed before the IO completion > in this case? The contract that callers of bvec iters need to hold their own references as without that doing I/O do them would be unsafe. It they did not hold references the pages could go away before even calling bio_iov_iter_get_pages (or this open coded bio_iov_bvec_set).