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

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 1/3/24 11:59, Hou Tao 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
> fuse_get_user_pages() only limits the length of fuse arg by max_read or
> max_write for IOV_KVEC io (e.g., kernel_read_file from finit_module()).
> For virtiofs, max_read is UINT_MAX, so a big read request which is about


I find this part of the explanation a bit confusing. I guess you wanted 
to write something like

fuse_direct_io() -> fuse_get_user_pages() is limited by 
fc->max_write/fc->max_read and fc->max_pages. For virtiofs max_pages 
does not apply as ITER_KVEC is used. As virtiofs sets fc->max_read to 
UINT_MAX basically no limit is applied at all.

I also wonder if it wouldn't it make sense to set a sensible limit in
virtio_fs_ctx_set_defaults() instead of introducing a new variable?

Also, I guess the issue is kmalloc_array() in virtio_fs_enqueue_req? 
Wouldn't it make sense to use kvm_alloc_array/kvfree in that function?


Thanks,
Bernd


> 10MB is passed to copy_args_to_argbuf(), kmalloc() is called in turn
> with len=10MB, and triggers the warning in __alloc_pages():
> WARN_ON_ONCE_GFP(order > MAX_ORDER, gfp)).
> 
> A feasible solution is to limit the value of max_read for virtiofs, so
> the length passed to kmalloc() will be limited. However it will affects
> the max read size for ITER_IOVEC io and the value of max_write also needs
> limitation. So instead of limiting the values of max_read and max_write,
> introducing max_nopage_rw to cap both the values of max_read and
> max_write when the fuse dio read/write request is initiated from kernel.
> 
> Considering that fuse read/write request from kernel is uncommon and to
> decrease the demand for large contiguous pages, set max_nopage_rw as
> 256KB instead of KMALLOC_MAX_SIZE - 4096 or similar.
> 
> Fixes: a62a8ef9d97d ("virtio-fs: add virtiofs filesystem")
> Signed-off-by: Hou Tao <houtao1@huawei.com>
> ---
>   fs/fuse/file.c      | 12 +++++++++++-
>   fs/fuse/fuse_i.h    |  3 +++
>   fs/fuse/inode.c     |  1 +
>   fs/fuse/virtio_fs.c |  6 ++++++
>   4 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index a660f1f21540..f1beb7c0b782 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1422,6 +1422,16 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
>   	return ret < 0 ? ret : 0;
>   }
>   
> +static size_t fuse_max_dio_rw_size(const struct fuse_conn *fc,
> +				   const struct iov_iter *iter, int write)
> +{
> +	unsigned int nmax = write ? fc->max_write : fc->max_read;
> +
> +	if (iov_iter_is_kvec(iter))
> +		nmax = min(nmax, fc->max_nopage_rw);
> +	return nmax;
> +}
> +
>   ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
>   		       loff_t *ppos, int flags)
>   {
> @@ -1432,7 +1442,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
>   	struct inode *inode = mapping->host;
>   	struct fuse_file *ff = file->private_data;
>   	struct fuse_conn *fc = ff->fm->fc;
> -	size_t nmax = write ? fc->max_write : fc->max_read;
> +	size_t nmax = fuse_max_dio_rw_size(fc, iter, write);
>   	loff_t pos = *ppos;
>   	size_t count = iov_iter_count(iter);
>   	pgoff_t idx_from = pos >> PAGE_SHIFT;
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 1df83eebda92..fc753cd34211 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -594,6 +594,9 @@ struct fuse_conn {
>   	/** Constrain ->max_pages to this value during feature negotiation */
>   	unsigned int max_pages_limit;
>   
> +	/** Maximum read/write size when there is no page in request */
> +	unsigned int max_nopage_rw;
> +
>   	/** Input queue */
>   	struct fuse_iqueue iq;
>   
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 2a6d44f91729..4cbbcb4a4b71 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -923,6 +923,7 @@ void fuse_conn_init(struct fuse_conn *fc, struct fuse_mount *fm,
>   	fc->user_ns = get_user_ns(user_ns);
>   	fc->max_pages = FUSE_DEFAULT_MAX_PAGES_PER_REQ;
>   	fc->max_pages_limit = FUSE_MAX_MAX_PAGES;
> +	fc->max_nopage_rw = UINT_MAX;
>   
>   	INIT_LIST_HEAD(&fc->mounts);
>   	list_add(&fm->fc_entry, &fc->mounts);
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 5f1be1da92ce..3aac31d45198 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -1452,6 +1452,12 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
>   	/* Tell FUSE to split requests that exceed the virtqueue's size */
>   	fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,
>   				    virtqueue_size - FUSE_HEADER_OVERHEAD);
> +	/* copy_args_to_argbuf() uses kmalloc-ed memory as bounce buffer
> +	 * for fuse args, so limit the total size of these args to prevent
> +	 * the warning in __alloc_pages() and decrease the demand for large
> +	 * contiguous pages.
> +	 */
> +	fc->max_nopage_rw = min(fc->max_nopage_rw, 256U << 10);
>   
>   	fsc->s_fs_info = fm;
>   	sb = sget_fc(fsc, virtio_fs_test_super, set_anon_super_fc);