Re: [PATCH RFC] vhost: don't use kmap() to log dirty pages

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 06, 2019 at 10:23:29PM -0400, Jason Wang wrote:
> Vhost log dirty pages directly to a userspace bitmap through GUP and
> kmap_atomic() since kernel doesn't have a set_bit_to_user()
> helper. This will cause issues for the arch that has virtually tagged
> caches. The way to fix is to keep using userspace virtual address.
> 
> Fortunately, futex has a cmpxchg to userspace memory helper
> futex_atomic_cmpxchg_inatomic(). So switch to use it to exchange the
> userspace bitmap with zero, set the bit and then write it back through
> put_user().
> 
> Note: there're archs (few non popular ones) that don't implement
> futex helper, we can't log dirty pages. We can fix them on top or
> simply disable LOG_ALL features of vhost.

Or implement futex_atomic_cmpxchg using kmap if they don't have
virtually tagged caches.

> 
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/vhost/vhost.c | 27 +++++++++++++++------------
>  1 file changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 351af88..9c94c41 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -31,6 +31,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/interval_tree_generic.h>
>  #include <linux/nospec.h>
> +#include <asm/futex.h>
>  
>  #include "vhost.h"
>  
> @@ -1692,25 +1693,27 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>  }
>  EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
>  
> -/* TODO: This is really inefficient.  We need something like get_user()
> - * (instruction directly accesses the data, with an exception table entry
> - * returning -EFAULT). See Documentation/x86/exception-tables.txt.
> - */
> -static int set_bit_to_user(int nr, void __user *addr)
> +static int set_bit_to_user(int nr, u32 __user *addr)
>  {
>  	unsigned long log = (unsigned long)addr;
>  	struct page *page;
> -	void *base;
> -	int bit = nr + (log % PAGE_SIZE) * 8;
> +	u32 old_log;
>  	int r;
>  
>  	r = get_user_pages_fast(log, 1, 1, &page);
>  	if (r < 0)
>  		return r;
>  	BUG_ON(r != 1);
> -	base = kmap_atomic(page);
> -	set_bit(bit, base);
> -	kunmap_atomic(base);
> +
> +	r = futex_atomic_cmpxchg_inatomic(&old_log, addr, 0, 0);
> +	if (r < 0)
> +		return r;

So I think this is a great idea!

However one issue here is that futex_atomic_cmpxchg_inatomic will fail if the
page is swapped out. I suspect we need a variant that blocks the thread
instead.

> +
> +	old_log |= 1 << nr;
> +	r = put_user(old_log, addr);
> +	if (r < 0)
> +		return r;
> +
>  	set_page_dirty_lock(page);
>  	put_page(page);
>  	return 0;
> @@ -1727,8 +1730,8 @@ static int log_write(void __user *log_base,
>  	write_length += write_address % VHOST_PAGE_SIZE;
>  	for (;;) {
>  		u64 base = (u64)(unsigned long)log_base;
> -		u64 log = base + write_page / 8;
> -		int bit = write_page % 8;
> +		u64 log = base + write_page / 32;
> +		int bit = write_page % 32;
>  		if ((u64)(unsigned long)log != log)
>  			return -EFAULT;
>  		r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
> -- 
> 1.8.3.1