Hi,
Can someone please explain the function set_pte_at to me? In
particular, I'm trying to understand the page fault handling in a PV
64-bit Linux.
Here's the sequence that I trace:
1) The Linux page fault handler at linux-2.6.18-xen.hg/mm/
memory.c:do_no_page runs.
2) After getting the page in memory, it calls set_pte_at.
3) set_pte_at has the following definition:
#define set_pte_at(_mm,addr,ptep,pteval) do {
if (((_mm) != current->mm && (_mm) != &init_mm) || \
HYPERVISOR_update_va_mapping((addr), (pteval), 0)) \
set_pte((ptep), (pteval)); \
In the case of a page fault, I think _mm will be equal to init_mm.
As a result, the first if condition fails, and we end up calling
HYPERVISOR_update_va_mapping. If that succeeds, then we're done.
Otherwise, we call set_pte which just sets the PTE directly. Since in
PV Linux, page tables don't have writeable mappings, this write causes
an instruction fault and Xen's ptwr_emulated_update tries to perform
the page table update.
So here are my questions:
1) What is the purpose of the first if condition ((_mm) != current->mm
&& (_mm) != &init_mm)? If the fault happens in a kernel virtual
memory context, why do we always want to call set_pte? Why don't we
just call HYPERVISOR_update_va_mapping? Does ptwr_emulated_update
handle a larger subset of the page table updates logic than
HYPERVISOR_update_va_mapping?
2) On a related note, if we do call HYPERVISOR_update_va_mapping and
it fails, why do we retry the page table update through
ptwr_emulated_update?
Thanks!
Mike
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