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    SubjectRe: [PATCH] mm: hugetlb: flush dcache before returning zeroed huge page to userspace
    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.

    >
    > Will

    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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