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Please forgive the following for deliberate simplifiations and accidental misunderstandings. If you flame gently, *maybe* we can pull something useful out of the ashen remains ;-) This is an evolution of my original plan to do per-cpu persisent kmap pools a couple of months ago - thanks to Rik for pointing out this was not going to work. OK, so currently we divide the virtual address space into user space (u-space) and kernel space (k-space) at the PAGE_OFFSET boundary, with two fundamental differences that I'm interested in for the sake of this argument. 1. u-space has different protections from k-space (ie user can't read/write it directly) 2. u-space is per task. k-space is common across all tasks. Imagine we create a hybrid "u-k-space" with the protections of k-space, but the locality of u-space .... either by making part of the current k-space per task or by making part of the current u-space protected like k-space ... not sure which would be easier. This u-k-space would be a good area for at least two things (and probably others): 1. A good place to put the process pagetables. We only use up the amount of virtual address space (vaddr space) for one task's pagetables - if we map them into ZONE_NORMAL (as current mainline) we use up vaddr space for *all* task's pagetables - if we map them through kmap (atomic or persistent), we pay dearly in tlbflushes. 2. A good place to make a per-task kmap area. This would be on a pool system similar to the current persistent kmap. We would potentially do only a local cpu tlb_flush_all when this table ran out (though if we're clever, we can use the context switches tlb_flush to do this for us). This would make copy_to_user stuff that's currently done under kmap cheaper. This, unfortunately, isn't a total solution - we may sometimes need to modify the task's pagetables from outside the process context, eg. swapout (thanks to dmc for pointing this out to me ;-)). For this, we'd just use the existing kmap mechanism to create another mapping to use temporarily, and we're no worse off than before. But on the whole I think it wins us enough to be worthwhile. Opinions? Martin. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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