Re: [PATCH] virtiofs: limit the length of ITER_KVEC dio by max_nopage_rw

["Michael S. Tsirkin" <mst@redhat.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