linux-kernel - Re: Problem with io

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Date:   Wed, 11 Oct 2023 13:21:30 +0200
From:   Sascha Hauer <sha@...gutronix.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        Boris Pismenny <borisp@...dia.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: Problem with io_uring splice and KTLS

On Tue, Oct 10, 2023 at 08:28:13AM -0600, Jens Axboe wrote:
> On 10/10/23 8:19 AM, Sascha Hauer wrote:
> > Hi,
> > 
> > I am working with a webserver using io_uring in conjunction with KTLS. The
> > webserver basically splices static file data from a pipe to a socket which uses
> > KTLS for encryption. When splice is done the socket is closed. This works fine
> > when using software encryption in KTLS. Things go awry though when the software
> > encryption is replaced with the CAAM driver which replaces the synchronous
> > encryption with a asynchronous queue/interrupt/completion flow.
> > 
> > So far I have traced it down to tls_push_sg() calling tcp_sendmsg_locked() to
> > send the completed encrypted messages. tcp_sendmsg_locked() sometimes waits for
> > more memory on the socket by calling sk_stream_wait_memory(). This in turn
> > returns -ERESTARTSYS due to:
> > 
> >         if (signal_pending(current))
> >                 goto do_interrupted;
> > 
> > The current task has the TIF_NOTIFY_SIGNAL set due to:
> > 
> > io_req_normal_work_add()
> > {
> >         ...
> >         /* This interrupts sk_stream_wait_memory() (notify_method == TWA_SIGNAL) */
> >         task_work_add(req->task, &tctx->task_work, ctx->notify_method)))
> > }
> > 
> > The call stack when sk_stream_wait_memory() fails is as follows:
> > 
> > [ 1385.428816]  dump_backtrace+0xa0/0x128
> > [ 1385.432568]  show_stack+0x20/0x38
> > [ 1385.435878]  dump_stack_lvl+0x48/0x60
> > [ 1385.439539]  dump_stack+0x18/0x28
> > [ 1385.442850]  tls_push_sg+0x100/0x238
> > [ 1385.446424]  tls_tx_records+0x118/0x1d8
> > [ 1385.450257]  tls_sw_release_resources_tx+0x74/0x1a0
> > [ 1385.455135]  tls_sk_proto_close+0x2f8/0x3f0
> > [ 1385.459315]  inet_release+0x58/0xb8
> > [ 1385.462802]  inet6_release+0x3c/0x60
> > [ 1385.466374]  __sock_release+0x48/0xc8
> > [ 1385.470035]  sock_close+0x20/0x38
> > [ 1385.473347]  __fput+0xbc/0x280
> > [ 1385.476399]  ____fput+0x18/0x30
> > [ 1385.479537]  task_work_run+0x80/0xe0
> > [ 1385.483108]  io_run_task_work+0x40/0x108
> > [ 1385.487029]  __arm64_sys_io_uring_enter+0x164/0xad8
> > [ 1385.491907]  invoke_syscall+0x50/0x128
> > [ 1385.495655]  el0_svc_common.constprop.0+0x48/0xf0
> > [ 1385.500359]  do_el0_svc_compat+0x24/0x40
> > [ 1385.504279]  el0_svc_compat+0x38/0x108
> > [ 1385.508026]  el0t_32_sync_handler+0x98/0x140
> > [ 1385.512294]  el0t_32_sync+0x194/0x198
> > 
> > So the socket is being closed and KTLS tries to send out the remaining
> > completed messages.  From a splice point of view everything has been sent
> > successfully, but not everything made it through KTLS to the socket and the
> > remaining data is sent while closing the socket.
> > 
> > I vaguely understand what's going on here, but I haven't got the
> > slightest idea what to do about this. Any ideas?
> 
> Two things to try:
> 
> 1) Depending on how you use the ring, set it up with
> IORING_SETUP_SINGLE_ISSUER | IORING_SETUP_DEFER_TASKRUN. The latter will
> avoid using signal based task_work notifications, which may be messing
> you up here.

These flags do not make a difference unfortunately.

> 
> 2) io_uring will hold a reference to the file/socket. I'm unsure if this
> is a problem in the above case, but sometimes it'll prevent the final
> flush.

Not sure what you want me to test here.

FWIW I tried to do the close() outside of io_uring and just did a
regular close() in userspace. That didn't make a difference either.

> 
> Do you have a reproducer that could be run to test? Sometimes easier to
> see what's going on when you can experiment, it'll save some time.

I would love to provide a reproducer, but you'll need a device with an
asynchronous encryption engine providing gcm(aes). I am using the CAAM
engine on a Layerscape board, but some i.MX6/8 based board should do
as well. I don't know what other hardware you might have which supports
that.

Sascha

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