Messages in this thread | | | Date | Fri, 25 Jan 2008 19:08:28 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH UPDATE] x86: ignore spurious faults |
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* Jeremy Fitzhardinge <jeremy@goop.org> wrote:
> Ingo Molnar wrote: >> thanks, applied. >> >> it would be nice to expose this ability of the architecture to the core >> Linux kernel mprotect code as well, and let it skip on a TLB flush when >> doing a RO->RW transition. > > The usermode fault handler already effectively does this; this patch > just does it for kernel mode as well. I don't know if mprotect takes > advantage of this.
spurious faults happen all the time on SMP, in the native kernel.
And what i mean is that Linux mprotect currently does not take advantage of x86's ability to just change the ptes, because there's no structured way to tell mm/mprotect.c that "it's safe to skip the TLB flush here".
The flush happens in mm/mprotect.c's change_protection() function:
flush_tlb_range(vma, start, end);
and that is unnecessary when we increase the protection rights, such as in a RO->RW change. (all that is needed is an smp_wmb() instead, to make sure all the pte modifications are visible when the syscall returns.)
and it's a really rare case these days that you can find an area where Linux does not make use of a hardware MMU feature - so we should fix this ;-)
Ingo
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