On Thu, Mar 18, 2010 at 20:10, Siddhartha Chhabra
<siddhartha.chhabra@gmail.com> wrote:
It depends.
If the kernel's doing a copy_from_user or copy_to_user family of calls
(ie, the calls used in system call handlers when accessing user space
buffers referenced in the arguments), this will trigger a page fault
exactly like the userspace process would, and the PF handler will then
deal with any copy on write or whatever may be needed. Of course, if a
userspace access wouldn't trigger a PF, the kernel access won't
either.
For the actual copy-on-write process itself, it would be a Bad Thing
to trigger a recursive page fault, so instead the kernel will directly
access the page via the direct mapped section of the address space -
this will never cause a PF (on x86, this may require creating a
temporary mapping for memory at a high physical address, but this
still won't be a PF as it will be set up before the first access).
Additionally, memory mapped IO involves direct DMA to/from pages that
are simultaneously in use by userspace - this won't cause a PF in
kernel mode either. Same with swap.
In short, some kernel accesses to user space do go through normal
channels which may or may not PF; other accesses will never PF. So
it's a bad idea to rely on all kernel accesses triggering a page
fault.
Hope this helps,
Bryan
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