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    Subject[Revert] Re: [PATCH] mm: sync vmalloc address space page tables in alloc_vm_area()
    On Thu, Sep 01, 2011 at 12:51:03PM +0100, David Vrabel wrote:
    > From: David Vrabel <david.vrabel@citrix.com>

    Andrew,

    I was wondering if you would be Ok with this patch for 3.1.

    It is a revert (I can prepare a proper revert if you would like
    that instead of this patch).

    The users of this particular function (alloc_vm_area) are just
    Xen. There are no others.

    >
    > Xen backend drivers (e.g., blkback and netback) would sometimes fail
    > to map grant pages into the vmalloc address space allocated with
    > alloc_vm_area(). The GNTTABOP_map_grant_ref would fail because Xen
    > could not find the page (in the L2 table) containing the PTEs it
    > needed to update.
    >
    > (XEN) mm.c:3846:d0 Could not find L1 PTE for address fbb42000
    >
    > netback and blkback were making the hypercall from a kernel thread
    > where task->active_mm != &init_mm and alloc_vm_area() was only
    > updating the page tables for init_mm. The usual method of deferring
    > the update to the page tables of other processes (i.e., after taking a
    > fault) doesn't work as a fault cannot occur during the hypercall.
    >
    > This would work on some systems depending on what else was using
    > vmalloc.
    >
    > Fix this by reverting ef691947d8a3d479e67652312783aedcf629320a
    > (vmalloc: remove vmalloc_sync_all() from alloc_vm_area()) and add a
    > comment to explain why it's needed.
    >
    > Signed-off-by: David Vrabel <david.vrabel@citrix.com>
    > Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
    > ---
    > mm/vmalloc.c | 8 ++++++++
    > 1 files changed, 8 insertions(+), 0 deletions(-)
    >
    > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
    > index 7ef0903..5016f19 100644
    > --- a/mm/vmalloc.c
    > +++ b/mm/vmalloc.c
    > @@ -2140,6 +2140,14 @@ struct vm_struct *alloc_vm_area(size_t size)
    > return NULL;
    > }
    >
    > + /*
    > + * If the allocated address space is passed to a hypercall
    > + * before being used then we cannot rely on a page fault to
    > + * trigger an update of the page tables. So sync all the page
    > + * tables here.
    > + */
    > + vmalloc_sync_all();
    > +
    > return area;
    > }
    > EXPORT_SYMBOL_GPL(alloc_vm_area);
    > --
    > 1.7.2.5


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