Re: [PATCH] mm: hugetlb: flush dcache before returning zeroed hugepage to userspace

[James Bottomley]

On Thu, 2012-07-12 at 13:16 +0200, Michal Hocko wrote:
> On Wed 11-07-12 18:48:02, Will Deacon wrote:
> > On Tue, Jul 10, 2012 at 11:42:34AM +0100, Will Deacon wrote:
> > > On Tue, Jul 10, 2012 at 10:45:13AM +0100, Will Deacon wrote:
> > > > On Tue, Jul 10, 2012 at 12:57:14AM +0100, Hugh Dickins wrote:
> > > > > If I start to grep the architectures for non-empty flush_dcache_page(),
> > > > > I soon find things in arch/arm such as v4_mc_copy_user_highpage() doing
> > > > > if (!test_and_set_bit(PG_dcache_clean,)) __flush_dcache_page() - where
> > > > > the naming suggests that I'm right, it's the architecture's responsibility
> > > > > to arrange whatever flushing is needed in its copy and clear page functions.
> > 
> > [...]
> > 
> > > Ok, so this is exactly the problem. The hugetlb allocator uses its own
> > > pool of huge pages, so free_huge_page followed by a later alloc_huge_page
> > > will give you something where the page flags of the compound head do not
> > > guarantee that PG_arch_1 is clear.
> > 
> > Just to confirm, the following quick hack at least results in the correct
> > flushing for me (on ARM):
> > 
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index e198831..7a7c9d3 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1141,6 +1141,7 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
> >         }
> >  
> >         set_page_private(page, (unsigned long)spool);
> > +       clear_bit(PG_arch_1, &page->flags);
> >  
> >         vma_commit_reservation(h, vma, addr);
> >  
> > 
> > 
> > The question is whether we should tidy that up for the core code or get
> > architectures to clear the bit in arch_make_huge_pte (which also seems to
> > work).
> 
> This should go into arch specific code IMO. Even the page flag name
> suggests this shouldn't be in the base code.

Agree completely.

PG_Arch_1 is mostly used for flushing implementations, but it's meaning
isn't unified.  For instance Arm and Parisc have opposite meanings for
this flag.  Touching it in generic code can therefore *never* be the
right thing to do.  What you're looking for here is the correct flushing
(or rather flush notification) interface.

James


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