lkml.org 
[lkml]   [2021]   [Mar]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] vfio/pci: Handle concurrent vma faults
On Thu, Mar 11, 2021 at 11:35:24AM +0000, Christoph Hellwig wrote:
> On Wed, Mar 10, 2021 at 03:06:07PM -0500, Peter Xu wrote:
> > On Wed, Mar 10, 2021 at 02:40:11PM -0400, Jason Gunthorpe wrote:
> > > On Wed, Mar 10, 2021 at 11:34:06AM -0700, Alex Williamson wrote:
> > >
> > > > > I think after the address_space changes this should try to stick with
> > > > > a normal io_rmap_pfn_range() done outside the fault handler.
> > > >
> > > > I assume you're suggesting calling io_remap_pfn_range() when device
> > > > memory is enabled,
> > >
> > > Yes, I think I saw Peter thinking along these lines too
> > >
> > > Then fault just always causes SIGBUS if it gets called
>
> I feel much more comfortable having the io_remap_pfn_range in place.

It's just that Jason convinced me with the fact that io_remap_pfn_range() will
modify vma flags, and I tend to agree that's not a good thing to do during a
fault() handler (in remap_pfn_range):

vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;

Although this case is special and it does not do harm it seems, since all these
four flags are already set by vfio_pci_mmap() anyways, so the flag didn't
really change at least with current code base. It's just still cleaner to not
use io_remap_pfn_range() in vfio fault() since future change to the function
io_remap_pfn_range() may not guarantee to match with vfio mmap().

>
> >
> > Indeed that looks better than looping in the fault().
> >
> > But I don't know whether it'll be easy to move io_remap_pfn_range() to device
> > memory enablement. If it's a two-step thing, we can fix the BUG_ON and vma
> > duplication issue first, then the full rework can be done in the bigger series
> > as what be chosen as the last approach.
>
> What kind of problems do you envision? It seems pretty simple to do,
> at least when combined with the unmap_mapping_range patch.

Moving the prefault into device memory enablement will even remove the 1st
fault delay when doing the first MMIO access that triggers this fault(). Also
in that case I think we can also call io_remap_pfn_range() directly and safely,
rather than looping over vmf_insert_pfn_prot().

Thanks,

--
Peter Xu

\
 
 \ /
  Last update: 2021-03-11 17:38    [W:0.113 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site