lkml.org 
[lkml]   [2012]   [Jul]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] mm: hugetlb: flush dcache before returning zeroed huge page to userspace
    On Mon, 9 Jul 2012, Will Deacon wrote:
    > On Mon, Jul 09, 2012 at 01:25:23PM +0100, Michal Hocko wrote:
    > > On Wed 04-07-12 15:32:56, Will Deacon wrote:
    > > > When allocating and returning clear huge pages to userspace as a
    > > > response to a fault, we may zero and return a mapping to a previously
    > > > dirtied physical region (for example, it may have been written by
    > > > a private mapping which was freed as a result of an ftruncate on the
    > > > backing file). On architectures with Harvard caches, this can lead to
    > > > I/D inconsistency since the zeroed view may not be visible to the
    > > > instruction stream.
    > > >
    > > > This patch solves the problem by flushing the region after allocating
    > > > and clearing a new huge page. Note that PowerPC avoids this issue by
    > > > performing the flushing in their clear_user_page implementation to keep
    > > > the loader happy, however this is closely tied to the semantics of the
    > > > PG_arch_1 page flag which is architecture-specific.
    > > >
    > > > Acked-by: Catalin Marinas <catalin.marinas@arm.com>
    > > > Signed-off-by: Will Deacon <will.deacon@arm.com>
    > > > ---
    > > > mm/hugetlb.c | 1 +
    > > > 1 files changed, 1 insertions(+), 0 deletions(-)
    > > >
    > > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
    > > > index e198831..b83d026 100644
    > > > --- a/mm/hugetlb.c
    > > > +++ b/mm/hugetlb.c
    > > > @@ -2646,6 +2646,7 @@ retry:
    > > > goto out;
    > > > }
    > > > clear_huge_page(page, address, pages_per_huge_page(h));
    > > > + flush_dcache_page(page);
    > > > __SetPageUptodate(page);
    > >
    > > Does this have to be explicit in the arch independent code?
    > > It seems that ia64 uses flush_dcache_page already in the clear_user_page
    >
    > It would match what is done in similar situations by cow_user_page (mm/memory.c)
    > and shmem_writepage (mm/shmem.c). Other subsystems also have explicit page
    > flushing (DMA bounce, ksm) so I think this is the right place for it.

    I am not at all sure if you are right or not:
    please let's consult linux-arch about this - now Cc'ed.

    If this hugetlb_no_page() were solely mapping the hugepage into that
    userspace, I would say you are wrong. It's the job of clear_huge_page()
    to take the mapped address into account, and pass it down to the
    architecture-specific implementation, to do whatever flushing is
    needed - you should be providing that in your architecture.

    In particular, notice how clear_huge_page() goes round a loop of
    clear_user_highpage()s: in your patch, you're expecting the implementation
    of flush_dcache_page() to notice whether or not this is a hugepage, and
    flush the appropriate size.

    Perhaps yours is the only architecture to need this on huge, and your
    flush_dcache_page() implements it correctly; but it does seem surprising.

    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.

    But... this hugetlb_no_page() has a VM_MAYSHARE case below, which puts
    the new page into page cache, making it accessible by other processes:
    that may indeed be reason for flush_dcache_page() there - or a loop of
    flush_dcache_page()s. But I worry then that in the !VM_MAYSHARE case
    you would be duplicating expensive flushes: perhaps they should be
    restricted to the VM_MAYSHARE block.

    Hugh


    \
     
     \ /
      Last update: 2012-07-10 02:41    [W:0.034 / U:0.012 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site