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    SubjectRe: [Revert] Re: [PATCH] mm: sync vmalloc address space page tables in alloc_vm_area()
    On 01/09/11 22:17, Andrew Morton wrote:
    > On Thu, 01 Sep 2011 13:37:46 -0700
    > Jeremy Fitzhardinge <jeremy@goop.org> wrote:
    >
    >> On 09/01/2011 09:11 AM, Konrad Rzeszutek Wilk wrote:
    >>> 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).
    >
    > David's patch looks better than a straight reversion.
    >
    > Problem is, I can't find David's original email anywhere. Someone's
    > been playing games with To: headers?

    Sorry, I should have Cc'd linux-kernel and others on the original patch.

    From 6844ca07140e08f29454ca7b3fa459571c7ba428 Mon Sep 17 00:00:00 2001
    From: David Vrabel <david.vrabel@citrix.com>
    Date: Thu, 1 Sep 2011 11:42:50 +0100
    Subject: [PATCH] mm: sync vmalloc address space page tables in
    alloc_vm_area()

    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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