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

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

On Fri, 2 Sep 2011 12:39:19 +0100
David Vrabel <david.vrabel@xxxxxxxxxx> 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?


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