Re: [PATCH 2/3] x86,mm: drop TLB flush from ptep_set_access_flags

On 11/17/2012 09:56 AM, Linus Torvalds wrote:

On Sat, Nov 17, 2012 at 6:50 AM, Borislav Petkov [off-list ref] wrote:

quoted

I don't know, however, whether it would be prudent to have some sort of
a cheap assertion in the code (cheaper than INVLPG %ADDR, although on
older cpus we do MOV CR3) just in case. This should be enabled only with
DEBUG_VM on, of course...

I wonder how we could actually test for it. We'd have to have some
per-cpu page-fault address check (along with a generation count on the
mm or similar). I doubt we'd figure out anything that works reliably
and efficiently and would actually show any problems

Would it be enough to simply print out a warning if we fault
on the same address twice (or three times) in a row, and then
flush the local TLB?

I realize this would not just trigger on CPUs that fail to
invalidate TLB entries that cause faults, but also on kernel
paths that cause a page fault to be re-taken...

... but then again, don't we want to find those paths and
fix them, anyway? :)

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