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 15:13:28 -0700 [thread overview]
Message-ID: <e0e574f0-9938-eaba-cffb-c66f754f33c5@kernel.dk> (raw)
In-Reply-To: <9db63405-6797-9305-3ce1-fdc11edbf49c@kernel.dk>
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
next prev parent reply other threads:[~2019-01-16 22:13 UTC|newest]
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
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 [this message]
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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