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[66.29.188.166]) by smtp.gmail.com with ESMTPSA id m85sm11111336pfj.171.2019.01.16.14.21.22 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 16 Jan 2019 14:21:24 -0800 (PST) Subject: Re: [PATCH 12/15] io_uring: add support for pre-mapped user IO buffers To: Dave Chinner 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 References: <20190116175003.17880-1-axboe@kernel.dk> <20190116175003.17880-13-axboe@kernel.dk> <20190116205338.GQ4205@dastard> <9db63405-6797-9305-3ce1-fdc11edbf49c@kernel.dk> <20190116220938.GR4205@dastard> From: Jens Axboe Message-ID: <7fd5cb40-2288-3c54-41d1-3163098b25ef@kernel.dk> Date: Wed, 16 Jan 2019 15:21:21 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190116220938.GR4205@dastard> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 1/16/19 3:09 PM, Dave Chinner wrote: > On Wed, Jan 16, 2019 at 02:20:53PM -0700, 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? > > get_user_pages_longterm() is the right thing to use to ensure DAX > doesn't trip over this - it's effectively just get_user_pages() > with a "if (vma_is_fsdax(vma))" check in it to abort and return > -EOPNOTSUPP. IOWs, this is safe on DAX but it's not safe on anything > else. :/ > > Unfortunately, disallowing userspace GUP pins on non-DAX file backed > pages will break existing "mostly just work" userspace apps all over > the place. And so right now there are discussions ongoing about how > to map gup references avoid the writeback races and be able to be > seen/tracked by other kernel infrastructure (see the long, long > thread "[PATCH 0/2] put_user_page*(): start converting the call > sites" on -fsdevel). Progress is slow, but I think we're starting to > close on a workable solution. > > FWIW, this doesn't solve the "long term user pin will block > filesystem operations until unpin" problem, that's what moving to > using revocable file layout leases is intended to solve. There have > been patches posted some time ago to add this user API for this, but > we've got to solve the other problems first.... > >> Would seem that most >> (all?) callers of this API is currently broken then. > > Yup, there's a long, long history of machines using userspace RDMA > panicing because filesystems have detected or tripped over invalid > page cache state during writeback attempts. This is not a new > problem.... Thanks for your detailed answer, Dave! I didn't see it before I sent out the previous email. FWIW, I've updated the patch: http://git.kernel.dk/cgit/linux-block/commit/?h=io_uring&id=0c8f2299f8069af6b2fa8f99a10d81646d1237a7 Checks for file backed memory, fails the registration with EOPNOTSUPP if the check fails. That should handle the issue on the io_uring side at least, and it's a restriction that can always be relaxed/lifted, when appropriate solutions to file backed buffers exists. -- Jens Axboe