Re: io_uring failure on parisc with VIPT caches

Helge Deller
2023-02-15T15:52:17Z

On 2/15/23 16:16, Jens Axboe wrote:
> On 2/14/23 7:12?PM, John David Anglin wrote:
>> On 2023-02-14 6:29 p.m., Jens Axboe wrote:
>>> On 2/14/23 4:09?PM, Helge Deller wrote:
>>>> * John David Anglin<dave.anglin@bell.net>:
>>>>> On 2023-02-13 5:05 p.m., Helge Deller wrote:
>>>>>> On 2/13/23 22:05, Jens Axboe wrote:
>>>>>>> On 2/13/23 1:59?PM, Helge Deller wrote:
>>>>>>>>> Yep sounds like it. What's the caching architecture of parisc?
>>>>>>>> parisc is Virtually Indexed, Physically Tagged (VIPT).
>>>>>>> That's what I assumed, so virtual aliasing is what we're dealing with
>>>>>>> here.
>>>>>>>
>>>>>>>> Thanks for the patch!
>>>>>>>> Sadly it doesn't fix the problem, as the kernel still sees
>>>>>>>> ctx->rings->sq.tail as being 0.
>>>>>>>> Interestingly it worked once (not reproduceable) directly after bootup,
>>>>>>>> which indicates that we at least look at the right address from kernel side.
>>>>>>>>
>>>>>>>> So, still needs more debugging/testing.
>>>>>>> It's not like this is untested stuff, so yeah it'll generally be
>>>>>>> correct, it just seems that parisc is a bit odd in that the virtual
>>>>>>> aliasing occurs between the kernel and userspace addresses too. At least
>>>>>>> that's what it seems like.
>>>>>> True.
>>>>>>
>>>>>>> But I wonder if what needs flushing is the user side, not the kernel
>>>>>>> side? Either that, or my patch is not flushing the right thing on the
>>>>>>> kernel side.
>>>> The patch below seems to fix the issue.
>>>>
>>>> I've successfuly tested it with the io_uring-test testcase on
>>>> physical parisc machines with 32- and 64-bit 6.1.11 kernels.
>>>>
>>>> The idea is similiar on how a file is mmapped shared by two
>>>> userspace processes by keeping the lower bits of the virtual address
>>>> the same.
>>>>
>>>> Cache flushes from userspace don't seem to be needed.
>>> Are they from the kernel side, if the lower bits mean we end up
>>> with the same coloring? Because I think this is a bit of a big
>>> hammer, in terms of overhead for flushing. As an example, on arm64
>>> that is perfectly fine with the existing code, it's about a 20-25%
>>> performance hit.
>>
>> The io_uring-test testcase still works on rp3440 with the kernel
>> flushes removed.
>
> That's what I suspected, the important bit here is just aligning it for
> identical coloring. Can you confirm if the below works for you? Had to
> fiddle it a bit to get it to work without coloring.

Yes, the patch works for me on 32- and 64-bit, even with PA8900 CPUs...

Is there maybe somewhere a more detailled testcase which I could try too?

Some nits below...

> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index db623b3185c8..1d4562067949 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -72,6 +72,7 @@
> #include <linux/io_uring.h>
> #include <linux/audit.h>
> #include <linux/security.h>
> +#include <asm/shmparam.h>
>
> #define CREATE_TRACE_POINTS
> #include <trace/events/io_uring.h>
> @@ -3200,6 +3201,51 @@ static __cold int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
> return remap_pfn_range(vma, vma->vm_start, pfn, sz, vma->vm_page_prot);
> }
>
> +static unsigned long io_uring_mmu_get_unmapped_area(struct file *filp,
> + unsigned long addr, unsigned long len,
> + unsigned long pgoff, unsigned long flags)
> +{
> + const unsigned long mmap_end = arch_get_mmap_end(addr, len, flags);
> + struct vm_unmapped_area_info info;
> + void *ptr;
> +
> + ptr = io_uring_validate_mmap_request(filp, pgoff, len);
> + if (IS_ERR(ptr))
> + return -ENOMEM;
> +
> + /* we do not support requesting a specific address */
> + if (addr)
> + return -EINVAL;

With this ^ we disallow users to provide a proposed address.
I think this is ok and I suggest to keep it that way.

Alternatively one could check the given address against the
alignment which is calculated below, but this will make the
code IMHO unnecessary bigger.

> +
> + info.flags = VM_UNMAPPED_AREA_TOPDOWN;
> + info.length = len;
> + info.low_limit = max(PAGE_SIZE, mmap_min_addr);
> + info.high_limit = arch_get_mmap_base(addr, current->mm->mmap_base);
> + info.align_mask = PAGE_MASK;
> + info.align_offset = (unsigned long) ptr;

For parisc I introduced SHM_COLOUR because it allows userspace
to map a shared file initially at any PAGE_SIZE-aligned address.
Only if then a second user maps the same file, the aliasing will be enforced.

Other platforms just have SHMLBA, and for some SHMLBA is > PAGE_SIZE.
So, instead of above code, this untested code might be better for those other
platforms ?
info.align_mask = PAGE_MASK & (SHMLBA - 1);
info.align_offset = (unsigned long)ptr & (SHMLBA - 1);

this is ok ->
> +#ifdef SHM_COLOUR
> + info.align_mask &= (SHM_COLOUR - 1);
> + info.align_offset &= (SHM_COLOUR - 1)

^^ misses a ";" at the end.

Helge

> +#endif
> +
> + /*
> + * A failed mmap() very likely causes application failure,
> + * so fall back to the bottom-up function here. This scenario
> + * can happen with large stack limits and large mmap()
> + * allocations.
> + */
> + addr = vm_unmapped_area(&info);
> + if (offset_in_page(addr)) {
> + VM_BUG_ON(addr != -ENOMEM);
> + info.flags = 0;
> + info.low_limit = TASK_UNMAPPED_BASE;
> + info.high_limit = mmap_end;
> + addr = vm_unmapped_area(&info);
> + }
> +
> + return addr;
> +}
> +
> #else /* !CONFIG_MMU */
>
> static int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
> @@ -3414,6 +3460,8 @@ static const struct file_operations io_uring_fops = {
> #ifndef CONFIG_MMU
> .get_unmapped_area = io_uring_nommu_get_unmapped_area,
> .mmap_capabilities = io_uring_nommu_mmap_capabilities,
> +#else
> + .get_unmapped_area = io_uring_mmu_get_unmapped_area,
> #endif
> .poll = io_uring_poll,
> #ifdef CONFIG_PROC_FS
>