Re: [PATCH 14/15] mm: fix TLB flush race between migration, andchange_protection_range

[Christoph Lameter]

On Fri, 6 Dec 2013, Rik van Riel wrote:
>
> The basic race looks like this:
>
> CPU A			CPU B			CPU C
>
> 						load TLB entry
> make entry PTE/PMD_NUMA
> 			fault on entry
> 						read/write old page
> 			start migrating page

When you start migrating a page a special page migration entry is
created that will trap all accesses to the page. You can safely flush when
the migration entry is there. Only allow a new PTE/PMD to be put there
*after* the tlb flush.


> 			change PTE/PMD to new page

Dont do that. We have migration entries for a reason.

> 						read/write old page [*]

Should cause a page fault which should put the process to sleep. Process
will safely read the page after the migration entry is removed.

> flush TLB

Establish the new PTE/PMD after the flush removing the migration pte
entry and thereby avoiding the race.

> 						reload TLB from new entry
> 						read/write new page
> 						lose data
>
> [*] the old page may belong to a new user at this point!
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/