Re: [Xen-devel] Re: [PATCH 0/6] xen: don't call vmalloc_sync_all() when mapping foreign pages

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 09/22/2011 04:06 AM, Stefano Stabellini wrote:
> On Wed, 21 Sep 2011, Jeremy Fitzhardinge wrote:
>> On 09/21/2011 03:42 AM, Stefano Stabellini wrote:
>>> On Thu, 15 Sep 2011, Jeremy Fitzhardinge wrote:
>>>> This series is relying on regular ram mappings are already synced to all
>>>> tasks, but I'm not sure that's necessarily guaranteed (for example, if
>>>> you hotplug new memory into the domain, the new pages won't be mapped
>>>> into every mm unless they're synced).
>>> the series is using GFP_KERNEL, so this problem shouldn't occur, right?
>> What properties do you think GFP_KERNEL guarantees?
> That the memory is below 4G and always mapped in the kernel 1:1 region.

Hm, but that's not quite the same thing as "mapped into every
pagetable".  Lowmem pages always have a kernel virtual address, and its
always OK to touch them at any point in kernel code[*] because one can
rely on the fault handler to create mappings as needed - but that
doesn't mean they're necessarily mapped by present ptes in the current
pagetable.

[*] - except NMI handlers

> Regarding memory hotplug it looks like that x86_32 is mapping new memory
> ZONE_HIGHMEM, therefore avoiding any problems with GFP_KERNEL allocations.
> On the other hand x86_64 is mapping the memory ZONE_NORMAL and calling
> init_memory_mapping on the new range right away. AFAICT changes to
> the 1:1 mapping in init_mm are automatically synced across all mm's
> because the pgd is shared?

TBH I'm not sure.  vmalloc_sync_one/all does seem to do *something* on
64-bit, but I was never completely sure what regions of the address
space were already shared.  I *think* it might be that the pgd and pud
are not shared, but the pmd down is, so if you add a new pmd you need to
sync it into all the puds (and puds into pgds if you add a new one of
those).

But I'd be happier pretending that vmalloc_sync_* just doesn't exist,
and deal with it at the hypercall level - in the short term, by just
making sure that the callers touch all those pages before passing them
into the hypercall.

    J