Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: [RFC] Page table sharing | Date | Wed, 20 Feb 2002 15:18:30 +0100 |
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On February 19, 2002 07:11 pm, Hugh Dickins wrote: > On Tue, 19 Feb 2002, Linus Torvalds wrote: > > On Tue, 19 Feb 2002, Daniel Phillips wrote: > > > > > > > > At that point you might as well make the TLB shootdown global (ie you keep > > > > track of a mask of CPU's whose TLB's you want to kill, and any pmd that > > > > has count > 1 just makes that mask be "all CPU's"). > > > > > > How do we know when to do the global tlb flush? > > > > See above. > > > > Basically, the algorithm is: > > > > invalidate_cpu_mask = 0; > > > > .. for each page swapped out .. > > > > pte = ptep_get_and_clear(ptep); > > save_pte_and_mm(pte_page(pte)); > > mask = mm->cpu_vm_mask; > > if (page_count(pmd_page) > 1) > > mask = ~0UL; > > invalidate_cpu_mask |= mask; > > > > and then at the end you just do > > > > flush_tlb_cpus(invalidate_cpu_mask); > > for_each_page_saved() { > > free_page(page); > > } > > > > (yeah, yeah, add cache coherency etc). > > It's a little worse than this, I think. Propagating pte_dirty(pte) to > set_page_dirty(page) cannot be done until after the flush_tlb_cpus,
You mean, because somebody might re-dirty an already cleaned page? Or are you driving at something more subtle?
> if the ptes are writable: and copy_page_range is not setting "cow", so not > write protecting, when it's a shared writable mapping. Easy answer is > to scrap "cow" there and always do the write protection; but I doubt > that's the correct answer.
Nope. For shared mmaps you'd get tons of unecessary faults.
> swap_out could keep an array of pointers to > ptes, to propagate dirty after flushing TLB and before freeing pages, > but that's not very pretty.
It's not horrible, not worse than the already-existing tlb_remove_page code anyway. I think we're not stopped here, just slowed down for some head scratching.
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