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    SubjectRe: [Revert] Re: [PATCH] mm: sync vmalloc address space page tables in alloc_vm_area()
    On 09/02/2011 03:32 PM, Andrew Morton wrote:
    > On Fri, 2 Sep 2011 12:39:19 +0100
    > David Vrabel <david.vrabel@citrix.com> wrote:
    >
    >> 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.
    > oookay, I queued this for 3.1 and tagged it for a 3.0.x backport. I
    > *think* that's the outcome of this discussion, for the short-term?


    Sure, that will get things going for now.

    Thanks,
    J



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