Re: [RFC PATCH V2] vhost: don't use kmap() to log dirty pages

[Jason Wang]

On 2019/5/9 äå9:18, Michael S. Tsirkin wrote:
> On Thu, May 09, 2019 at 08:58:00AM -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 arch_futex_atomic_op_inuser() which
> > could be used for setting a bit to 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 e.g for non
> >    virtually tagged archs implement a kmap fallback on top or simply
> >    disable LOG_ALL features of vhost.
> > - The helper also requires userspace pointer is located at 4-byte
> >    boundary, need to check during dirty log setting
> Why check? Round it down.


Will do this.


> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> > Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
> > Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> > Changes from V1:
> > - switch to use arch_futex_atomic_op_inuser()
> > ---
> >   drivers/vhost/vhost.c | 35 +++++++++++++++++------------------
> >   1 file changed, 17 insertions(+), 18 deletions(-)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 351af88..4e5a004 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"
> >   
> > @@ -1652,6 +1653,10 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> >   			r = -EFAULT;
> >   			break;
> >   		}
> > +		if (p & 0x3) {
> > +			r = -EINVAL;
> > +			break;
> > +		}
> >   		for (i = 0; i < d->nvqs; ++i) {
> >   			struct vhost_virtqueue *vq;
> >   			void __user *base = (void __user *)(unsigned long)p;
> That's an ABI change and might break some userspace. I don't think
> it's necessary: you are changing individual bits anyway.


Right.


> > @@ -1692,31 +1697,27 @@ 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;
> >   	int r;
> >   
> >   	r = get_user_pages_fast(log, 1, 1, &page);
> OK so the trick is that page is pinned so you don't expect
> arch_futex_atomic_op_inuser below to fail.  get_user_pages_fast
> guarantees page is not going away but does it guarantee PTE won't be
> invaidated or write protected?


Good point, then I think we probably need to do manual fixup through 
fixup_user_fault() if arch_futex_atomic_op_in_user() fail.


> >   	if (r < 0)
> >   		return r;
> >   	BUG_ON(r != 1);
> > -	base = kmap_atomic(page);
> > -	set_bit(bit, base);
> > -	kunmap_atomic(base);
> > +
> > +	r = arch_futex_atomic_op_inuser(FUTEX_OP_ADD, 1 << nr, &old, addr);
> > +	/* TODO: fallback to kmap() when -ENOSYS? */
> > +
> Add a comment why this won't fail? Maybe warn on EFAULT?
>
> Also down the road a variant that does not need tricks like this is
> still nice to have.


Ok. Let me post a V3.

Thanks


> >   	set_page_dirty_lock(page);
> >   	put_page(page);
> > -	return 0;
> > +	return r;
> >   }
> >   
> > -static int log_write(void __user *log_base,
> > +static int log_write(u32 __user *log_base,
> >   		     u64 write_address, u64 write_length)
> >   {
> >   	u64 write_page = write_address / VHOST_PAGE_SIZE;
> > @@ -1726,12 +1727,10 @@ static int log_write(void __user *log_base,
> >   		return 0;
> >   	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;
> > -		if ((u64)(unsigned long)log != log)
> > -			return -EFAULT;
> > -		r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
> > +		u32 __user *log = log_base + write_page / 32;
> > +		int bit = write_page % 32;
> > +
> > +		r = set_bit_to_user(bit, log);
> >   		if (r < 0)
> >   			return r;
> >   		if (write_length <= VHOST_PAGE_SIZE)
> > --
> > 1.8.3.1