Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: Note describing poor dcache utilization under high memory pressure | Date | Tue, 29 Jan 2002 02:45:46 +0100 |
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On January 29, 2002 02:29 am, Oliver Xymoron wrote: > 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.
Yes, that's an essential detail I omitted: when a page table's use count transitions from 1 to 2, mark all the CoW pages on the page table RO.
> If they're not, then either parent or child writing to > non-faulting addresses will be writing to shared memory.
Yes, and after all, the whole point is to generalize CoW of pages to include instantiation of page tables.
> 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.
Oh yes, it's what I intended, thanks. Um, and I think you just told me what one of my bugs is.
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