Re: [PATCH] mm: migrate: add missing flush_dcache_page for non-mapped page migrate

[Mel Gorman]

On Tue, Feb 19, 2019 at 01:32:12PM +0100, Lars Persson wrote:
> Our MIPS 1004Kc SoCs were seeing random userspace crashes with SIGILL
> and SIGSEGV that could not be traced back to a userspace code
> bug. They had all the magic signs of an I/D cache coherency issue.
> 
> Now recently we noticed that the /proc/sys/vm/compact_memory interface
> was quite efficient at provoking this class of userspace crashes.
> 
> Studying the code in mm/migrate.c there is a distinction made between
> migrating a page that is mapped at the instant of migration and one
> that is not mapped. Our problem turned out to be the non-mapped pages.
> 
> For the non-mapped page the code performs a copy of the page content
> and all relevant meta-data of the page without doing the required
> D-cache maintenance. This leaves dirty data in the D-cache of the CPU
> and on the 1004K cores this data is not visible to the I-cache. A
> subsequent page-fault that triggers a mapping of the page will happily
> serve the process with potentially stale code.
> 
> What about ARM then, this bug should have seen greater exposure? Well
> ARM became immune to this flaw back in 2010, see commit c01778001a4f
> ("ARM: 6379/1: Assume new page cache pages have dirty D-cache").
> 
> My proposed fix moves the D-cache maintenance inside move_to_new_page
> to make it common for both cases.
> 
> Signed-off-by: Lars Persson <larper@xxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

-- 
Mel Gorman
SUSE Labs