Re: [Xen-devel] What's the purpose of VM_FOREIGN

["Andrew Warfield" <andrew.warfield@xxxxxxxxxxxx>]

VM_FOREIGN is used to allow zero-copy of data pages for the blktap
driver.  When foreign block data pages are mapped to user space, the
PTEs aren't backed by page structs, so get_user_pages becomes upset
when called to map the block requests down onto physical page
addresses.

VM_FOREIGN vm_areas use the vm_private_data field to contain an array
of mappings to the associate page structs, and get_user_pages uses
this table to handle pages that have been foreign mapped.  The code
that sets up the mapping is in drivers/xen/blktap.c.

a.

On 8/1/06, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
> > But I never saw when the bit is set, so this code path can not be
> > reached?
> >
>
> It's used by the blktap driver.
>
>   -- Keir
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