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

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

On 1/10/24 02:16, Hou Tao wrote:
> Hi,
> 
> On 1/9/2024 9:11 PM, Bernd Schubert wrote:
>>
>>
>> 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.
> 
> Yes, what you said is just as expected but it is not the root cause of
> the warning. The culprit of the warning is kmalloc() in
> copy_args_to_argbuf() just as said in commit message. vmalloc() is also
> not acceptable, because the physical memory needs to be contiguous. For
> the problem, because there is no page involved, so there will be extra
> sg available, maybe we can use these sg to break the big read/write
> request into page.

Hmm ok, I was hoping that contiguous memory is not needed.
I see that ENOMEM is handled, but how that that perform (or even 
complete) on a really badly fragmented system? I guess splitting into 
smaller pages or at least adding some reserve kmem_cache (or even 
mempool) would make sense?

>>
>> 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?
> 
> As said in the commit message:
> 
> 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.
> 
> It is a bit hard to set a reasonable value for both max_read and
> max_write to handle both normal ITER_IOVEC io and ITER_KVEC io. And
> considering ITER_KVEC io + dio case is uncommon, I think using a new
> limitation is more reasonable.

For ITER_IOVEC max_pages applies - which is limited to 
FUSE_MAX_MAX_PAGES - why can't this be used in virtio_fs_ctx_set_defaults?

@Miklos, is there a reason why there is no upper fc->max_{read,write} 
limit in process_init_reply()? Shouldn't both be limited to
(FUSE_MAX_MAX_PAGES * PAGE_SIZE). Or any other reasonable limit?


Thanks,
Bernd



>>
>> 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);
>> .
> 
>