From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-block-owner@vger.kernel.org>
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 627BFC678D4
	for <linux-block@archiver.kernel.org>; Thu, 19 Jan 2023 00:15:59 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S229570AbjASAP5 (ORCPT <rfc822;linux-block@archiver.kernel.org>);
        Wed, 18 Jan 2023 19:15:57 -0500
Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41862 "EHLO
        lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S229606AbjASAP4 (ORCPT
        <rfc822;linux-block@vger.kernel.org>);
        Wed, 18 Jan 2023 19:15:56 -0500
Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [IPv6:2a03:a000:7:0:5054:ff:fe1c:15ff])
        by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 03CEE5355F;
        Wed, 18 Jan 2023 16:15:55 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed;
        d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type:
        MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To:
        Content-Transfer-Encoding:Content-ID:Content-Description;
        bh=B4JKTopwf+ah1JzO+o8lKdNMaw0A3fc5NXU+YNHQFBU=; b=RQ+fXNP99ggIbZtBXxqO4wTlTl
        5hJSTT5zs8PR1I7e3wiqq5aC8sq0ARV3Xp7QGdkSLljcjiLdfklaz5WAs6eCPn49gFu0N9VQqFXDM
        oM05bDhNraV2tKr1XSZfPIazui4XxePY0SFJyITfItPI/5d9a59kxoo7qCK/suhZiDtunkbg3fHJC
        SOF8gaSn70LL+c97Xei2fWI03cNHSqga8PhaRRmTeyEEgPjx28qXC+1IlM9o5WaX9P4BcmAZ2F6+5
        uOzCs5+WZFkgTV3bBj65ecJsnymTxkrNBXqvxxZxMbo8mcEB45B2nqHOLJFr5A93nMKWKbnKG/2uo
        UyOdFPkA==;
Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux))
        id 1pIIam-002dwf-2G;
        Thu, 19 Jan 2023 00:15:44 +0000
Date:   Thu, 19 Jan 2023 00:15:44 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     David Howells <dhowells@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v6 03/34] iov_iter: Pass I/O direction into
 iov_iter_get_pages*()
Message-ID: <Y8iLsPlSLy5YVffX@ZenIV>
References: <167391047703.2311931.8115712773222260073.stgit@warthog.procyon.org.uk>
 <167391050409.2311931.7103784292954267373.stgit@warthog.procyon.org.uk>
 <Y8ZU1Jjx5VSetvOn@infradead.org>
 <Y8h62KsnI8g/xaRz@ZenIV>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Y8h62KsnI8g/xaRz@ZenIV>
Sender: Al Viro <viro@ftp.linux.org.uk>
Precedence: bulk
List-ID: <linux-block.vger.kernel.org>
X-Mailing-List: linux-block@vger.kernel.org

On Wed, Jan 18, 2023 at 11:03:52PM +0000, Al Viro wrote:
> On Mon, Jan 16, 2023 at 11:57:08PM -0800, Christoph Hellwig wrote:
> > On Mon, Jan 16, 2023 at 11:08:24PM +0000, David Howells wrote:
> > > Define FOLL_SOURCE_BUF and FOLL_DEST_BUF to indicate to get_user_pages*()
> > > and iov_iter_get_pages*() how the buffer is intended to be used in an I/O
> > > operation.  Don't use READ and WRITE as a read I/O writes to memory and
> > > vice versa - which causes confusion.
> > > 
> > > The direction is checked against the iterator's data_source.
> > 
> > Why can't we use the existing FOLL_WRITE?
> 
> 	I'm really not fond of passing FOLL_... stuff into iov_iter
> primitives.  That space contains things like FOLL_PIN, which makes
> no sense whatsoever for non-user-backed iterators; having the
> callers pass it in makes them automatically dependent upon the
> iov_iter flavour.

Actually, looking at that thing...  Currently we use it only for
FOLL_PCI_P2PDMA.  It alters behaviour of get_user_pages_fast(), but...
it is completely ignored for ITER_BVEC or ITER_PIPE.  So how the
hell is it supposed to work?

And ITER_BVEC *can* get there.  blkdev_direct_IO() can get anything
->write_iter() can get, and io_uring will feed stuff to it.  For
that matter, ->read_iter() can lead to it as well, so
generic_file_splice_read() can end up passing ITER_PIPE to that
sucker.

Could somebody give a braindump on that thing?  It looks like we
have pages that should not be DMA'd to/from unless driver takes
some precautions and we want to make sure they won't be fed to
drivers that don't take such.  With checks done in a very odd
place...