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From: Jens Axboe <axboe@kernel.dk>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
	linux-block@vger.kernel.org, linux-arch@vger.kernel.org,
	hch@lst.de, jmoyer@redhat.com, avi@scylladb.com
Subject: Re: [PATCH 12/15] io_uring: add support for pre-mapped user IO buffers
Date: Wed, 16 Jan 2019 14:20:53 -0700
Message-ID: <9db63405-6797-9305-3ce1-fdc11edbf49c@kernel.dk> (raw)
In-Reply-To: <20190116205338.GQ4205@dastard>

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(&current->mm->mmap_sem);
>> +		pret = get_user_pages_longterm(ubuf, nr_pages, FOLL_WRITE,
>> +						pages, NULL);
>> +		up_write(&current->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


  reply index

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-16 17:49 [PATCHSET v5] io_uring IO interface Jens Axboe
2019-01-16 17:49 ` [PATCH 01/15] fs: add an iopoll method to struct file_operations Jens Axboe
2019-01-16 17:49 ` [PATCH 02/15] block: wire up block device iopoll method Jens Axboe
2019-01-16 17:49 ` [PATCH 03/15] block: add bio_set_polled() helper Jens Axboe
2019-01-16 17:49 ` [PATCH 04/15] iomap: wire up the iopoll method Jens Axboe
2019-01-16 17:49 ` [PATCH 05/15] Add io_uring IO interface Jens Axboe
2019-01-17 12:02   ` Roman Penyaev
2019-01-17 13:54     ` Jens Axboe
2019-01-17 14:34       ` Roman Penyaev
2019-01-17 14:54         ` Jens Axboe
2019-01-17 15:19           ` Roman Penyaev
2019-01-17 12:48   ` Roman Penyaev
2019-01-17 14:01     ` Jens Axboe
2019-01-17 20:03       ` Jeff Moyer
2019-01-17 20:09         ` Jens Axboe
2019-01-17 20:14           ` Jens Axboe
2019-01-17 20:50             ` Jeff Moyer
2019-01-17 20:53               ` Jens Axboe
2019-01-17 21:02                 ` Jeff Moyer
2019-01-17 21:17                   ` Jens Axboe
2019-01-17 21:21                     ` Jeff Moyer
2019-01-17 21:27                       ` Jens Axboe
2019-01-18  8:23               ` Roman Penyaev
2019-01-16 17:49 ` [PATCH 06/15] io_uring: add fsync support Jens Axboe
2019-01-16 17:49 ` [PATCH 07/15] io_uring: support for IO polling Jens Axboe
2019-01-16 17:49 ` [PATCH 08/15] fs: add fget_many() and fput_many() Jens Axboe
2019-01-16 17:49 ` [PATCH 09/15] io_uring: use fget/fput_many() for file references Jens Axboe
2019-01-16 17:49 ` [PATCH 10/15] io_uring: batch io_kiocb allocation Jens Axboe
2019-01-16 17:49 ` [PATCH 11/15] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-01-16 17:50 ` [PATCH 12/15] io_uring: add support for pre-mapped user IO buffers Jens Axboe
2019-01-16 20:53   ` Dave Chinner
2019-01-16 21:20     ` Jens Axboe [this message]
2019-01-16 22:09       ` Dave Chinner
2019-01-16 22:21         ` Jens Axboe
2019-01-16 23:09           ` Dave Chinner
2019-01-16 23:17             ` Jens Axboe
2019-01-16 22:13       ` Jens Axboe
2019-01-16 17:50 ` [PATCH 13/15] io_uring: add submission polling Jens Axboe
2019-01-16 17:50 ` [PATCH 14/15] io_uring: add file registration Jens Axboe
2019-01-16 17:50 ` [PATCH 15/15] io_uring: add io_uring_event cache hit information Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2019-01-10  2:43 [PATCHSET v2] io_uring IO interface Jens Axboe
2019-01-10  2:44 ` [PATCH 12/15] io_uring: add support for pre-mapped user IO buffers Jens Axboe

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