Re: Note describing poor dcache utilization under high memory pressure

[Oliver Xymoron]

On Mon, 28 Jan 2002, Daniel Phillips wrote:

>   copy_page_range
>     Intead of copying the page tables, just increment their use counts
>
>   zap_page_range:
>     If a page table is shared according to its use count, just decrement
>     the use count and otherwise leave it alone.
>
>   handle_mm_fault:
>     If a page table is shared according to its use count and the faulting
>     instruction is a write, allocate a new page table and do the work that
>     would have normally been done by copy_page_range at fork time.
>     Decrement the use count of the (perhaps formerly) shared page table.

Somewhere in here, the pages have got to all be marked read-only or
something. If they're not, then either parent or child writing to
non-faulting addresses will be writing to shared memory.

I think something more is needed, such as creating a minimal page table
for the child process with read-only mappings to the current %EIP and %EBP
pages in it. This gets us past the fork/exec hurdle. Without the exec, we
copy over chunks when they're accessed as above in handle_mm_fault. But
you can't actually _share_ the page tables without marking the pages
themselves readonly.

-- 
 "Love the dolphins," she advised him. "Write by W.A.S.T.E.."