On 1/16/19 2:20 PM, Jens Axboe wrote:
> On 1/16/19 1:53 PM, Dave Chinner wrote:
>> On Wed, Jan 16, 2019 at 10:50:00AM -0700, Jens Axboe wrote:
>>> If we have fixed user buffers, we can map them into the kernel when we
>>> setup the io_context. That avoids the need to do get_user_pages() for
>>> each and every IO.
>> .....
>>> + return -ENOMEM;
>>> + } while (atomic_long_cmpxchg(&ctx->user->locked_vm, cur_pages,
>>> + new_pages) != cur_pages);
>>> +
>>> + return 0;
>>> +}
>>> +
>>> +static int io_sqe_buffer_unregister(struct io_ring_ctx *ctx)
>>> +{
>>> + int i, j;
>>> +
>>> + if (!ctx->user_bufs)
>>> + return -EINVAL;
>>> +
>>> + for (i = 0; i < ctx->sq_entries; i++) {
>>> + struct io_mapped_ubuf *imu = &ctx->user_bufs[i];
>>> +
>>> + for (j = 0; j < imu->nr_bvecs; j++) {
>>> + set_page_dirty_lock(imu->bvec[j].bv_page);
>>> + put_page(imu->bvec[j].bv_page);
>>> + }
>>
>> Hmmm, so we call set_page_dirty() when the gup reference is dropped...
>>
>> .....
>>
>>> +static int io_sqe_buffer_register(struct io_ring_ctx *ctx, void __user *arg,
>>> + unsigned nr_args)
>>> +{
>>
>> .....
>>
>>> + down_write(¤t->mm->mmap_sem);
>>> + pret = get_user_pages_longterm(ubuf, nr_pages, FOLL_WRITE,
>>> + pages, NULL);
>>> + up_write(¤t->mm->mmap_sem);
>>
>> Thought so. This has the same problem as RDMA w.r.t. using
>> file-backed mappings for the user buffer. It is not synchronised
>> against truncate, hole punches, async page writeback cleaning the
>> page, etc, and so can lead to data corruption and/or kernel panics.
>>
>> It also can't be used with DAX because the above problems are
>> actually a user-after-free of storage space, not just a dangling
>> page reference that can be cleaned up after the gup pin is dropped.
>>
>> Perhaps, at least until we solve the GUP problems w.r.t. file backed
>> pages and/or add and require file layout leases for these reference,
>> we should error out if the user buffer pages are file-backed
>> mappings?
>
> Thanks for taking a look at this.
>
> I'd be fine with that restriction, especially since it can get relaxed
> down the line. Do we have an appropriate API for this? And why isn't
> get_user_pages_longterm() that exact API already? Would seem that most
> (all?) callers of this API is currently broken then.
I guess for now I can just pass in a vmas array for
get_user_pages_longeterm() and then iterate those and check for
vma->vm_file. If it's set, then we fail the buffer registration.
And then drop the set_page_dirty() on release, we don't need that.
--
Jens Axboe
