On Fri, Feb 23, 2024 at 10:42:37AM +0100, Miklos Szeredi wrote: > On Wed, 3 Jan 2024 at 11:58, Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote: > > > > From: Hou Tao <houtao1@xxxxxxxxxx> > > > > 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
