From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1753257AbcK0Cvr (ORCPT <rfc822;w@1wt.eu>);
        Sat, 26 Nov 2016 21:51:47 -0500
Received: from zeniv.linux.org.uk ([195.92.253.2]:42000 "EHLO
        ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1752755AbcK0Cvm (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Sat, 26 Nov 2016 21:51:42 -0500
Date: Sun, 27 Nov 2016 02:51:18 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>, Yan@ZenIV.linux.org.uk,
        Zheng <zyan@redhat.com>, Zhu@ZenIV.linux.org.uk,
        Caifeng <zhucaifeng@unissoft-nj.com>
Subject: Re: [git pull] vfs fix
Message-ID: <20161127025113.GX1555@ZenIV.linux.org.uk>
References: <20161127011352.GV1555@ZenIV.linux.org.uk>
 <CA+55aFxnB+VMEcAfPA5H17LxOZCHBACD7kUQkY7tCAmVLkq5DQ@mail.gmail.com>
 <20161127022506.GW1555@ZenIV.linux.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161127022506.GW1555@ZenIV.linux.org.uk>
User-Agent: Mutt/1.7.1 (2016-10-04)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sun, Nov 27, 2016 at 02:25:09AM +0000, Al Viro wrote:

> Anyway, leaving that BUG_ON() had been wrong; I can send a followup
> massaging that thing as you've suggested, if you are interested in
> that.  But keep in mind that the whole iov_iter_get_pages...() calling
> conventions are going to be changed, hopefully soon.

BTW, speaking of iov_iter_get_pages_alloc() callers and splice:
"ceph: combine as many iovec as possile into one OSD request" has added
static size_t dio_get_pagev_size(const struct iov_iter *it)
{
    const struct iovec *iov = it->iov;
    const struct iovec *iovend = iov + it->nr_segs;
    size_t size;

    size = iov->iov_len - it->iov_offset;
    /*
     * An iov can be page vectored when both the current tail
     * and the next base are page aligned.
     */
    while (PAGE_ALIGNED((iov->iov_base + iov->iov_len)) &&
           (++iov < iovend && PAGE_ALIGNED((iov->iov_base)))) {
        size += iov->iov_len;
    }
    dout("dio_get_pagevlen len = %zu\n", size);
    return size;
}

... with 'it' possibly being bio_vec-backed iterator.  Could somebody
explain what that code is trying to do?

I would really like to hear details - I'm not saying that the goal mentioned
in the commit message is worthless, but I don't know ceph codebase well
enough to tell what would be a good way to implement that.  In its current
form it's obviously broken.