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

[Ilya Dryomov]

On Thu, Jan 12, 2017 at 12:53 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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