linux-kernel - Re: [git pull] vfs fix

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Message-ID: <20161127025113.GX1555@ZenIV.linux.org.uk>
Date:   Sun, 27 Nov 2016 02:51:18 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Yan@...IV.linux.org.uk, Zheng <zyan@...hat.com>,
        Zhu@...IV.linux.org.uk, Caifeng <zhucaifeng@...ssoft-nj.com>
Subject: Re: [git pull] vfs fix

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.