Received-SPF: pass (google.com: domain of linux-kernel+bounces-80018-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:40f1:3f00::1 as permitted sender) client-ip=2604:1380:40f1:3f00::1;
Date: Sun, 25 Feb 2024 03:46:16 -0500
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Hou Tao <houtao@huaweicloud.com>, linux-fsdevel@vger.kernel.org,
	Vivek Goyal <vgoyal@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>, linux-kernel@vger.kernel.org,
	virtualization@lists.linux.dev, houtao1@huawei.com
Subject: Re: [PATCH] virtiofs: limit the length of ITER_KVEC dio by
 max_nopage_rw
Message-ID: <20240225034356-mutt-send-email-mst@kernel.org>
References: <20240103105929.1902658-1-houtao@huaweicloud.com>
 <CAJfpegsM2ViQb1A2HNMJLsgVDs1UScd7p04MOLSkSMRNeshm0A@mail.gmail.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAJfpegsM2ViQb1A2HNMJLsgVDs1UScd7p04MOLSkSMRNeshm0A@mail.gmail.com>

On Fri, Feb 23, 2024 at 10:42:37AM +0100, Miklos Szeredi wrote:
> On Wed, 3 Jan 2024 at 11:58, Hou Tao <houtao@huaweicloud.com> wrote:
> >
> > From: Hou Tao <houtao1@huawei.com>
> >
> > When trying to insert a 10MB kernel module kept in a virtiofs with cache
> > disabled, the following warning was reported:
> >
> >   ------------[ cut here ]------------
> >   WARNING: CPU: 2 PID: 439 at mm/page_alloc.c:4544 ......
> >   Modules linked in:
> >   CPU: 2 PID: 439 Comm: insmod Not tainted 6.7.0-rc7+ #33
> >   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), ......
> >   RIP: 0010:__alloc_pages+0x2c4/0x360
> >   ......
> >   Call Trace:
> >    <TASK>
> >    ? __warn+0x8f/0x150
> >    ? __alloc_pages+0x2c4/0x360
> >    __kmalloc_large_node+0x86/0x160
> >    __kmalloc+0xcd/0x140
> >    virtio_fs_enqueue_req+0x240/0x6d0
> >    virtio_fs_wake_pending_and_unlock+0x7f/0x190
> >    queue_request_and_unlock+0x58/0x70
> >    fuse_simple_request+0x18b/0x2e0
> >    fuse_direct_io+0x58a/0x850
> >    fuse_file_read_iter+0xdb/0x130
> >    __kernel_read+0xf3/0x260
> >    kernel_read+0x45/0x60
> >    kernel_read_file+0x1ad/0x2b0
> >    init_module_from_file+0x6a/0xe0
> >    idempotent_init_module+0x179/0x230
> >    __x64_sys_finit_module+0x5d/0xb0
> >    do_syscall_64+0x36/0xb0
> >    entry_SYSCALL_64_after_hwframe+0x6e/0x76
> >    ......
> >    </TASK>
> >   ---[ end trace 0000000000000000 ]---
> >
> > The warning happened as follow. In copy_args_to_argbuf(), virtiofs uses
> > kmalloc-ed memory as bound buffer for fuse args, but
> 
> So this seems to be the special case in fuse_get_user_pages() when the
> read/write requests get a piece of kernel memory.
> 
> I don't really understand the comment in virtio_fs_enqueue_req():  /*
> Use a bounce buffer since stack args cannot be mapped */
> 
> Stefan, can you explain?  What's special about the arg being on the stack?

virtio core wants DMA'able addresses.

See Documentation/core-api/dma-api-howto.rst :

..


This rule also means that you may use neither kernel image addresses
(items in data/text/bss segments), nor module image addresses, nor
stack addresses for DMA.


> What if the arg is not on the stack (as is probably the case for big
> args like this)?   Do we need the bounce buffer in that case?
> 
> Thanks,
> Miklos