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.
--
Jens Axboe
