Re: [PATCHSET 0/5] User mapped provided buffer rings

Jens Axboe
2023-03-17T02:17:11Z

On 3/16/23 8:09?PM, Jens Axboe wrote:
> On 3/16/23 1:46?PM, Jens Axboe wrote:
>> On 3/16/23 1:08?PM, John David Anglin wrote:
>>> On 2023-03-15 5:18 p.m., Jens Axboe wrote:
>>>> On 3/15/23 2:38?PM, Jens Axboe wrote:
>>>>> On 3/15/23 2:07?PM, Helge Deller wrote:
>>>>>> On 3/15/23 21:03, Helge Deller wrote:
>>>>>>> Hi Jens,
>>>>>>>
>>>>>>> Thanks for doing those fixes!
>>>>>>>
>>>>>>> On 3/14/23 18:16, Jens Axboe wrote:
>>>>>>>> One issue that became apparent when running io_uring code on parisc is
>>>>>>>> that for data shared between the application and the kernel, we must
>>>>>>>> ensure that it's placed correctly to avoid aliasing issues that render
>>>>>>>> it useless.
>>>>>>>>
>>>>>>>> The first patch in this series is from Helge, and ensures that the
>>>>>>>> SQ/CQ rings are mapped appropriately. This makes io_uring actually work
>>>>>>>> there.
>>>>>>>>
>>>>>>>> Patches 2..4 are prep patches for patch 5, which adds a variant of
>>>>>>>> ring mapped provided buffers that have the kernel allocate the memory
>>>>>>>> for them and the application mmap() it. This brings these mapped
>>>>>>>> buffers in line with how the SQ/CQ rings are managed too.
>>>>>>>>
>>>>>>>> I'm not fully sure if this ONLY impacts archs that set SHM_COLOUR,
>>>>>>>> of which there is only parisc, or if SHMLBA setting archs (of which
>>>>>>>> there are others) are impact to any degree as well...
>>>>>>> It would be interesting to find out. I'd assume that other arches,
>>>>>>> e.g. sparc, might have similiar issues.
>>>>>>> Have you tested your patches on other arches as well?
>>>>>> By the way, I've now tested this series on current git head on an
>>>>>> older parisc box (with PA8700 / PCX-W2 CPU).
>>>>>>
>>>>>> Results of liburing testsuite:
>>>>>> Tests timed out (1): <send-zerocopy.t> - (may not be a failure)
>>>>>> Tests failed (5): <buf-ring.t> <file-verify.t> <poll-race-mshot.t> <ringbuf-read.t> <send_recvmsg.t>
>>>> If you update your liburing git copy, switch to the ring-buf-alloc branch,
>>>> then all of the above should work:
>>> With master liburing branch, test/poll-race-mshot.t still crashes my rp3440:
>>> Running test poll-race-mshot.t Bad cqe res -233
>>> Bad cqe res -233
>>> Bad cqe res -233
>>>
>>> There is a total lockup with no messages of any kind.
>>>
>>> I think the io_uring code needs to reject user supplied ring buffers that are not equivalently mapped
>>> to the corresponding kernel pages. Don't know if it would be possible to reallocate kernel pages so they
>>> are equivalently mapped.
>>
>> We can do that, you'd just want to add that check in io_pin_pbuf_ring()
>> when the pages have been mapped AND we're on an arch that has those
>> kinds of requirements. Maybe something like the below, totally
>> untested...
>>
>> I am puzzled where the crash is coming from, though. It should just hit
>> the -ENOBUFS case as it can't find a buffer, and that'd terminate that
>> request. Which does seem to be what is happening above, that is really
>> no different than an attempt to read/receive from a buffer group that
>> has no buffers available. So a bit puzzling on what makes your kernel
>> crash after that has happened, as we do have generic test cases that
>> exercise that explicitly.
>>
>>
>> diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
>> index cd1d9dddf58e..73f290aca7f1 100644
>> --- a/io_uring/kbuf.c
>> +++ b/io_uring/kbuf.c
>> @@ -491,6 +491,15 @@ static int io_pin_pbuf_ring(struct io_uring_buf_reg *reg,
>> return PTR_ERR(pages);
>>
>> br = page_address(pages[0]);
>> +#ifdef SHM_COLOUR
>> + if ((reg->ring_addr & (unsigned long) br) & SHM_COLOUR) {
>
> & (SHM_COLOUR - 1)) {
>
> of course...

Full version, I think this should do the right thing. If the kernel and
app side isn't aligned on the same SHM_COLOUR boundary, we'll return
-EINVAL rather than setup the ring.

For the ring-buf-alloc branch, this is handled automatically. But we
should, as you mentioned, ensure that the kernel doesn't allow setting
something up that will not work.

Note that this is still NOT related to your hang, I honestly have no
idea what that could be. Unfortunately parisc doesn't have a lot of
debugging aids for this... Could even be a generic kernel issue. I
looked up your rp3440, and it sounds like we have basically the same
setup. I'm running a dual socket PA8900 at 1GHz.

diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
index cd1d9dddf58e..7c6544456f90 100644
--- a/io_uring/kbuf.c
+++ b/io_uring/kbuf.c
@@ -491,6 +491,15 @@ static int io_pin_pbuf_ring(struct io_uring_buf_reg *reg,
return PTR_ERR(pages);

br = page_address(pages[0]);
+#ifdef SHM_COLOUR
+ if ((reg->ring_addr | (unsigned long) br) & (SHM_COLOUR - 1)) {
+ int i;
+
+ for (i = 0; i < nr_pages; i++)
+ unpin_user_page(pages[i]);
+ return -EINVAL;
+ }
+#endif
bl->buf_pages = pages;
bl->buf_nr_pages = nr_pages;
bl->buf_ring = br;

--
Jens Axboe