[Xen-devel] [Revert] Re: [PATCH] mm: sync vmalloc address space page tables in alloc_vm_area()

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Sep 01, 2011 at 12:51:03PM +0100, David Vrabel wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>

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@xxxxxxxxxx>
> Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
> ---
>  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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