Messages in this thread | | | Subject | Re: [PATCH RFC] vhost: don't use kmap() to log dirty pages | From | Jason Wang <> | Date | Wed, 8 May 2019 12:28:42 +0800 |
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On 2019/5/8 下午12:12, Michael S. Tsirkin wrote: > 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.
Yes, this might work.
> >> 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.
I guess not since the patch still try to pin the page before.
Thanks
> >> + >> + 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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