Messages in this thread | | | Date | Sun, 30 Apr 2006 19:29:53 +0200 | From | blaisorblade@yahoo ... | Subject | [patch 00/14] remap_file_pages protection support |
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Again (about 8 month since last time, I have much less time during my academic year), I'm sending for review (and for possible inclusion into -mm) protection support for remap_file_pages, i.e. setting per-pte protections (beyond file offset) through this syscall.
== How it works ==
Protections are set in the page tables when the page is loaded, are saved into the PTE when the page is swapped out and restored when the page is faulted back in.
Additionally, we modify the fault handler since the VMA protections aren't valid for PTE with modified protections.
Finally, we must also provide, for each arch, macros to store also the protections into the PTE; to make the kernel compile for any arch, I've added since last time dummy default macros to keep the same functionality.
== What is this for ==
The first idea is to use this for UML - it must create a lot of single page mappings, and managing them through separate VMAs is slow.
Additional note: this idea, with some further refinements (which I'll code after this chunk is accepted), will allow to reduce the number of used VMAs for most userspace programs - in particular, it will allow to avoid creating one VMA for one guard pages (which has PROT_NONE) - forcing PROT_NONE on that page will be enough.
This will be useful since the VMA lookup at fault time can be a bottleneck for some programs (I've received a report about this from Ulrich Drepper and I've been told that also Val Henson from Intel is interested about this). I guess that since we use RB-trees, the slowness is also due to the poor cache locality of RB-trees (since RB nodes are within VMAs but aren't accessed together with their content), compared for instance with radix trees where the lookup has high cache locality (but they have however space usage problems, possibly bigger, on 64-bit machines).
== Notes ==
Implementations are provided for i386, x86_64 and UML, and for some other archs I have patches I will send, based on the ones which were in -mm when Ingo sent the first version of this work.
You shouldn't worry for the number of patches, most of them are very little. I've last tested them in UML against 2.6.16-rc3, but I've seen no big changes in the VM. -- Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!". Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894) http://www.user-mode-linux.org/~blaisorblade - 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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