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

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