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    SubjectRe: [PATCH v2] ceph/iov_iter: fix bad iov_iter handling in ceph splice codepaths
    On Thu, Jan 12, 2017 at 8:59 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
    > On Tue, Jan 10, 2017 at 07:57:31AM -0500, Jeff Layton wrote:
    >> v2: fix bug in offset handling in iov_iter_pvec_size
    >>
    >> xfstest generic/095 triggers soft lockups in kcephfs. Basically it uses
    >> fio to drive some I/O via vmsplice ane splice. Ceph then ends up trying
    >> to access an ITER_BVEC type iov_iter as a ITER_IOVEC one. That causes it
    >> to pick up a wrong offset and get stuck in an infinite loop while trying
    >> to populate the page array. dio_get_pagev_size has a similar problem.
    >>
    >> To fix the first problem, add a new iov_iter helper to determine the
    >> offset into the page for the current segment and have ceph call that.
    >> I would just replace dio_get_pages_alloc with iov_iter_get_pages_alloc,
    >> but that will only return a single page at a time for ITER_BVEC and
    >> it's better to make larger requests when possible.
    >>
    >> For the second problem, we simply replace it with a new helper that does
    >> what it does, but properly for all iov_iter types.
    >>
    >> Since we're moving that into generic code, we can also utilize the
    >> iterate_all_kinds macro to simplify this. That means that we need to
    >> rework the logic a bit since we can't advance to the next vector while
    >> checking the current one.
    >
    > Yecchhh... That really looks like exposing way too low-level stuff instead
    > of coming up with saner primitive ;-/
    >
    > Is page vector + offset in the first page + number of bytes really what
    > ceph wants? Would e.g. an array of bio_vec be saner? Because _that_
    > would make a lot more natural iov_iter_get_pages_alloc() analogue...
    >
    > And yes, I realize that you have ->pages wired into the struct ceph_osd_request;
    > how painful would it be to have it switched to struct bio_vec array instead?

    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.

    That said, Jeff's patch doesn't look too bad to me...

    Thanks,

    Ilya

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