Re: [PATCH] vfio/type1: remove hugepage checks in is_invalid_reserved_pfn()

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 18 Oct 2019 14:42:32 +0800
Ben Luo <luoben@linux.alibaba.com> wrote:

> A friendly reminder :)
> 

Thanks Ben!  I've added this to the vfio next branch for v5.5 with
Andrea's R-b.  Thanks,

Alex

> 在 2019/10/4 上午12:41, Andrea Arcangeli 写道:
> > On Thu, Oct 03, 2019 at 11:49:42AM +0800, Ben Luo wrote:  
> >> Currently, no hugepage split code can transfer the reserved bit
> >> from head to tail during the split, so checking the head can't make
> >> a difference in a racing condition with hugepage spliting.
> >>
> >> The buddy wouldn't allow a driver to allocate an hugepage if any
> >> subpage is reserved in the e820 map at boot, if any driver sets the
> >> reserved bit of head page before mapping the hugepage in userland,
> >> it needs to set the reserved bit in all subpages to be safe.
> >>
> >> Signed-off-by: Ben Luo <luoben@linux.alibaba.com>  
> > Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> >
> >  
> >> ---
> >>   drivers/vfio/vfio_iommu_type1.c | 26 ++++----------------------
> >>   1 file changed, 4 insertions(+), 22 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index 054391f..e2019ba 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -287,31 +287,13 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
> >>    * Some mappings aren't backed by a struct page, for example an mmap'd
> >>    * MMIO range for our own or another device.  These use a different
> >>    * pfn conversion and shouldn't be tracked as locked pages.
> >> + * For compound pages, any driver that sets the reserved bit in head
> >> + * page needs to set the reserved bit in all subpages to be safe.
> >>    */
> >>   static bool is_invalid_reserved_pfn(unsigned long pfn)
> >>   {
> >> -	if (pfn_valid(pfn)) {
> >> -		bool reserved;
> >> -		struct page *tail = pfn_to_page(pfn);
> >> -		struct page *head = compound_head(tail);
> >> -		reserved = !!(PageReserved(head));
> >> -		if (head != tail) {
> >> -			/*
> >> -			 * "head" is not a dangling pointer
> >> -			 * (compound_head takes care of that)
> >> -			 * but the hugepage may have been split
> >> -			 * from under us (and we may not hold a
> >> -			 * reference count on the head page so it can
> >> -			 * be reused before we run PageReferenced), so
> >> -			 * we've to check PageTail before returning
> >> -			 * what we just read.
> >> -			 */
> >> -			smp_rmb();
> >> -			if (PageTail(tail))
> >> -				return reserved;
> >> -		}
> >> -		return PageReserved(tail);
> >> -	}
> >> +	if (pfn_valid(pfn))
> >> +		return PageReserved(pfn_to_page(pfn));
> >>   
> >>   	return true;
> >>   }
> >> -- 
> >> 1.8.3.1
> >>