Re: [PATCH v2] mm: re-allow pinning of zero pfns (again)

[Alex Williamson]

On Sat, 27 Aug 2022 17:59:32 -0700
John Hubbard <jhubbard@xxxxxxxxxx> wrote:

> On 8/10/22 09:53, Alex Williamson wrote:
> > The below referenced commit makes the same error as 1c563432588d ("mm: fix
> > is_pinnable_page against a cma page"), re-interpreting the logic to exclude
> > pinning of the zero page, which breaks device assignment with vfio.
> > 
> > To avoid further subtle mistakes, split the logic into discrete tests.
> > 
> > Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Suggested-by: Felix Kuehling <felix.kuehling@xxxxxxx>
> > Link: https://lore.kernel.org/all/165490039431.944052.12458624139225785964.stgit@omen
> > Fixes: f25cbb7a95a2 ("mm: add zone device coherent type memory support")
> > Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > ---
> >  include/linux/mm.h |   17 ++++++++++++++---
> >  1 file changed, 14 insertions(+), 3 deletions(-)  
> Hi Alex,
> 
> Looks good. I'm suggesting a simpler comment, below, because
> even though the VFIO folks are thinking about VFIO, here we
> are deep in the mm layer and there are lots of non-VFIO callers
> that may pin the zero page.
> 
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 18e01474cf6b..835106a9718f 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1544,9 +1544,20 @@ static inline bool is_longterm_pinnable_page(struct page *page)
> >  	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
> >  		return false;
> >  #endif
> > -	return !(is_device_coherent_page(page) ||
> > -		 is_zone_movable_page(page) ||
> > -		 is_zero_pfn(page_to_pfn(page)));
> > +	/*
> > +	 * The zero page might reside in a movable zone, however it may not
> > +	 * be migrated and can therefore be pinned.  The vfio subsystem pins
> > +	 * user mappings including the zero page for IOMMU translation.
> > +	 */  
> 
> Those notes are all about (some of) the callers. But it's a simple
> answer, really, so how about just this:
> 
> 	/* The zero page is always allowed to be pinned. */

Sure.  Are we looking for a re-spin with this?  I see Andrew already
added this incremental change to his hotfix-unstable branch separately.
I'd hate for a comment re-spin to delay getting a fix for this problem,
that blocks any VM use cases of VFIO, into mainline any longer.  Thanks,

Alex