linux-kernel - Re: [Revert] Re: [PATCH] mm: sync vmalloc address space page tables in alloc_vm

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Message-Id: <20110902153204.59a928c1.akpm@linux-foundation.org>
Date:	Fri, 2 Sep 2011 15:32:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"namhyung@...il.com" <namhyung@...il.com>,
	"rientjes@...gle.com" <rientjes@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [Revert] Re: [PATCH] mm: sync vmalloc address space page tables
 in alloc_vm_area()

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

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