Messages in this thread |  | | Subject | Re: [PATCH net] vhost: don't use kmap() to log dirty pages | From | Jason Wang <> | Date | Wed, 15 May 2019 10:55:35 +0800 |
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On 2019/5/15 上午6:16, Michael S. Tsirkin wrote: > On Mon, May 13, 2019 at 01:27:45AM -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 several cases that futex helper can fail e.g a page >> fault or the arch that doesn't have the support. For those cases, a >> simplified get_user()/put_user() pair protected by a global mutex is >> provided as a fallback. The fallback may lead false positive that >> userspace may see more dirty pages. >> >> Cc: Christoph Hellwig<hch@infradead.org> >> Cc: James Bottomley<James.Bottomley@HansenPartnership.com> >> Cc: Andrea Arcangeli<aarcange@redhat.com> >> Cc: Thomas Gleixner<tglx@linutronix.de> >> Cc: Ingo Molnar<mingo@redhat.com> >> Cc: Peter Zijlstra<peterz@infradead.org> >> Cc: Darren Hart<dvhart@infradead.org> >> Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server") >> Signed-off-by: Jason Wang<jasowang@redhat.com> >> --- >> Changes from RFC V2: >> - drop GUP and provide get_user()/put_user() fallbacks >> - round down log_base >> Changes from RFC V1: >> - switch to use arch_futex_atomic_op_inuser() >> --- >> drivers/vhost/vhost.c | 54 ++++++++++++++++++++++++++++----------------------- >> 1 file changed, 30 insertions(+), 24 deletions(-) >> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >> index 351af88..7fa05ba 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" >> >> @@ -43,6 +44,8 @@ >> MODULE_PARM_DESC(max_iotlb_entries, >> "Maximum number of iotlb entries. (default: 2048)"); >> >> +static DEFINE_MUTEX(vhost_log_lock); >> + >> enum { >> VHOST_MEMORY_F_LOG = 0x1, >> }; >> @@ -1692,28 +1695,31 @@ 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); >> - if (r < 0) >> - return r; >> - BUG_ON(r != 1); >> - base = kmap_atomic(page); >> - set_bit(bit, base); >> - kunmap_atomic(base); >> - set_page_dirty_lock(page); >> - put_page(page); >> + r = arch_futex_atomic_op_inuser(FUTEX_OP_OR, 1 << nr, &old, addr); >> + if (r) { >> + /* Fallback through get_user()/put_user(), this may >> + * lead false positive that userspace may see more >> + * dirty pages. A mutex is used to synchronize log >> + * access between vhost threads. >> + */ >> + mutex_lock(&vhost_log_lock); >> + r = get_user(old, addr); >> + if (r) >> + goto err; >> + r = put_user(old | 1 << nr, addr); >> + if (r) >> + goto err; >> + mutex_unlock(&vhost_log_lock); >> + } > Problem is, we always said it's atomic. > > This trick will work if userspace only clears bits > in the log, but won't if it sets bits in the log. > E.g. reusing the log structure for vhost-user > will have exactly this effect. >
Ok, I admit this is an issue.
Then I think maybe we can simply fallback to a u8 put_user() which is guaranteed to be atomic like:
put_user(0xF, addr);
Then we may at most have 7 more dirty pages to be seen by guest. Consider futex helper is implemented is most architectures and it's likely to succeed. It should be acceptable.
Does this make sense?
Thanks
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