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[66.29.188.166]) by smtp.gmail.com with ESMTPSA id h69sm8067796pge.4.2019.01.16.14.13.29 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 16 Jan 2019 14:13:30 -0800 (PST) Subject: Re: [PATCH 12/15] io_uring: add support for pre-mapped user IO buffers From: Jens Axboe 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> Message-ID: Date: Wed, 16 Jan 2019 15:13:28 -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: <9db63405-6797-9305-3ce1-fdc11edbf49c@kernel.dk> 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 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