Re: [PATCH 07/11] unpaged: COW on VM_UNPAGED

[Hugh Dickins <hugh@veritas.com>]

On Fri, 18 Nov 2005, David S. Miller wrote:
> 
> The recent vbetool suspend-from-ram datapoint shows that it might be
> important that the BIOS data area is application local,
> ie. MAP_PRIVATE.  Ie. it works only if the writes are not performed
> to the real BIOS data page.

Interesting.  I haven't dared reach that conclusion yet.

> If true, that means the MAP_PRIVATE+VM_UNPAGED case has legit users.
> Although, such applications could just copy the interrupt vector plus
> BIOS data area into an anonymously mapped region and have the vm86
> execution work off that instead of the /dev/mem mapping.

Yes, they could indeed.  Would save the kernel contortions.

> So, just to make sure this all adds up, a PROT_WRITE+MAP_PRIVATE
> mapping of /dev/mem results in any pages written to being COW'd.
> Right?

Yes, in everything before 2.6.15-rc1, and again with my patches.

> It is a good question as to which cases doing stuff like this want to
> make modifications to the real BIOS data area, and which ones do not.
> Aparently vbetool does not.

Hugh