> On Jul 18, 2016, at 07:43, Benjamin Coddington <bcodding@xxxxxxxxxx> wrote:
>
> On 11 Jul 2016, at 9:28, Trond Myklebust wrote:
>
>>> On Jul 11, 2016, at 08:59, Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
>>>
>>> We have a customer who was able to reliably reproduce the following hang:
>>> (hang itself is rare but there are many machines, so it is not rare)
>>>
>>> INFO: task ascp:66692 blocked for more than 120 seconds.
>>>
>>>> bt 66692
>>> PID: 66692 TASK: ffff883f124ba280 CPU: 5 COMMAND: "ascp"
>>>
>>> __schedule
>>> schedule
>>> schedule_timeout
>>> io_schedule_timeout
>>> io_schedule
>>> nfs_wait_bit_uninterruptible
>>> __wait_on_bit
>>> out_of_line_wait_on_bit
>>> nfs_wait_on_request
>>> nfs_try_to_update_request
>>> nfs_setup_write_request
>>> nfs_writepage_setup
>>> nfs_updatepage
>>> nfs_write_end
>>> generic_file_buffered_write
>>> __generic_file_aio_write
>>> generic_file_aio_write
>>> nfs_file_write
>>> do_sync_write
>>> vfs_write
>>> sys_write
>>> system_call_fastpath
>>>
>>> ascp is Aspera secure copy program. It is multithreaded.
>>> When the hang happens, 5 threads are in S-state and 1 on D-state.
>>>
>>> The workload is copying ~100+ GB files over NFS3 with 10GiB ethernet
>>> with the following mount options:
>>> rw,relatime,vers=3,rsize=131072,wsize=524288,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=X.X.X.X,mountvers=3,mountport=300,mountproto=udp,local_lock=none,addr=X.X.X.X
>>>
>>> We got coredump (2 actually) and observations are below:
>>>
>>> 1) one of the inode's pagecache pages has page->private available:
>>> struct nfs_page {
>>> wb_list = {
>>> next = 0xffff885623ac4f80,
>>> prev = 0xffff885623ac4f80
>>> },
>>> wb_page = 0xffffea01218c2600,
>>> wb_context = 0xffff887f2265de00,
>>> wb_lock_context = 0xffff887f2265de00,
>>> wb_index = 2649328,
>>> wb_offset = 0,
>>> wb_pgbase = 0,
>>> wb_bytes = 0,
>>> wb_kref = {
>>> refcount = {
>>> counter = 3
>>> }
>>> },
>>> wb_flags = 19,
>>> wb_verf = {
>>> data = "\000\000\000\000\000\000\000"
>>> },
>>> wb_this_page = 0xffff885623ac4f80,
>>> wb_head = 0xffff885623ac4f80
>>> }
>>>
>>> ->wb_list is always empty,
>>> ->wb_bytes is always 0 (!).
>>> ->wb_kref is always 3.
>>> ->wb_flags = PG_BUSY|PG_MAPPED|PG_INODE_REF,
>>>
>>> page->flags = 0x2869 =
>>> PG_writeback|PG_private|PG_active|PF_lru|PG_uptodate|PG_locked
>>>
>>> The zero-length request is created at nfs_create_request():
>>> WARN_ON_ONCE(req->wb_bytes == 0)
>>> nfs_create_request
>>> nfs_setup_write_request
>>> nfs_updatepage
>>> nfs_write_end
>>> generic_perform_write
>>> generic_file_buffered_write
>>> __generic_file_aio_write
>>> generic_file_aio_write
>>> nfs_file_write
>>> do_sync_write
>>> vfs_write
>>> SyS_write
>>>
>>> with count=0 coming from generic_perform_write (where else?).
>>>
>>> Customer is saying that RHEL6 was totally OK, and there are several reports
>>> of other people hitting same bug with RHEL7:
>>> https://gist.github.com/Millnert/ecc10d8cc79c81b55d7f
>>> https://bugs.centos.org/view.php?id=9284
>>>
>>
>> Why is this being reported here and not to Red Hat? Is the bug reproducible on the upstream kernel?
>
> There's a RHEL BZ open for it:
> https://bugzilla.redhat.com/show_bug.cgi?id=132463
>
> Upstream has the problem, too. The pgio layer doesn't expect zero-length
> requests, so an nfs_page can get locked but never added to a pageio
> descriptor.
>
> To create such a problem, writeback has to happen just after nfs_write_end()
> has created a nfs_page with wb_bytes = 0. This can happen if
> iov_iter_copy_from_user_atomic() fails to copy pages in
> generic_perform_write(). Normally, we'd just go around the loop and try the
> copy again, but if writeback sneaks in then the request gets locked but
> never added to a pgio descriptor due to the way we use conditions on zero
> bytes to move requests around.
>
> I see two potential ways to fix:
> - Just skip the creation of the request if the length is zero. We're
> bound to just come around and created it again.
>
> - Allow zero-length requests, but fix up __nfs_pageio_add_request,
> nfs_generic_pg_test, and others that use conditions on zero.
>
> I think the first option to handle the case early is the simpler fix, and
> best approach, since: what do we do with a zero-length request? Not
> creating them helps us be conservative in what we send out to the
> nfs_pageio_ops.
>
> I don't understand what conditions can cause iov_iter_copy_from_user_atomic
> to fail since pagefaults are disabled. Can anyone enlighten me?
>
> To reproduce this upstream, I've simulated iov_iter_copy_from_user_atomic()
> returning zero once, then inserted a small delay after nfs_write_end to
> allow me to sneak in a sync.
>
> Any advice on how to proceed? I'll send a patch to fix the first way
> otherwise. If anyone has trouble with that RHEL BZ, send me your bugzilla
> email and I'll give you access.
I think adding a check to nfs_updatepage() to bail out if ‘count’ is zero would seem to be the right thing to do.
Thanks
Trond
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