linux-kernel - Re: [PATCH v2] ceph/iov_iter: fix bad iov_iter handling in ceph splice codepaths

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Message-ID: <CAOi1vP-njp6HKScRuZxx5VY0u0iFiR-Ez7hTKpx0FSS90Kvc4g@mail.gmail.com>
Date:   Thu, 12 Jan 2017 13:17:05 +0100
From:   Ilya Dryomov <idryomov@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jeff Layton <jlayton@...hat.com>, "Yan, Zheng" <zyan@...hat.com>,
        Sage Weil <sage@...hat.com>,
        Ceph Development <ceph-devel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Zhu, Caifeng" <zhucaifeng@...ssoft-nj.com>
Subject: Re: [PATCH v2] ceph/iov_iter: fix bad iov_iter handling in ceph
 splice codepaths

On Thu, Jan 12, 2017 at 12:53 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Jan 12, 2017 at 12:46:42PM +0100, Ilya Dryomov wrote:
>> On Thu, Jan 12, 2017 at 12:37 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> > On Thu, Jan 12, 2017 at 12:13:31PM +0100, Ilya Dryomov wrote:
>> >
>> >> It would be a significant and wide-reaching change, but I've been
>> >> meaning to look into switching to iov_iter for a couple of releases
>> >> now.  There is a lot of ugly code in net/ceph/messenger.c to hangle
>> >> iteration over "page vectors", "page lists" and "bio lists".  All of it
>> >> predates iov_iter proliferation and is mostly incomplete anyway: IIRC
>> >> you can send out of a pagelist but can't recv into a pagelist, etc.
>> >
>> > Wait a sec...  Is it done from the same thread that has issued a syscall?
>> > If so, we certainly could just pass iov_iter without bothering with any
>> > form of ..._get_pages(); if not, we'll need at least to get from iovec
>> > to bio_vec, since userland addresses make sense only in the caller's
>> > context...
>>
>> No, not necessarily - it's also used by rbd (all of net/ceph has two
>> users: fs/ceph and drivers/block/rbd.c).
>
> Yes, but rbd doesn't deal with userland-pointing iovecs at all, does it?

Correct.  Normal I/O is all bios + currently it uses some of that
custom page vector/list machinery for maintenance operations (rbd
metadata, locks, etc).  No userland pointers whatsoever for rbd, but
the messenger (checksumming + {send,recv}{msg,page}) runs out of the
kworker for both rbd and cephfs.

Thanks,

                Ilya