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

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Message-ID: <20170112113743.GX1555@ZenIV.linux.org.uk>
Date:   Thu, 12 Jan 2017 11:37:43 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Ilya Dryomov <idryomov@...il.com>
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 12:13:31PM +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...