Re: [PATCH] no-execute -- please test

Albert Cahalan writes:

The first is to avoid taking the initial fault on the segment
which contains the instruction pointer.

Good idea.

The second is to avoid cache or TLB invalidates or flushes
in certain circumstances. OpenBSD developers report that
this type of optimization is of great benefit on sparc and ppc.
It's an optimization than is only valid when no-execute is
in use, so the performance effects go both ways.

We have a bit per page that says if the page is icache dirty or not.
On machines with no-execute support, we already avoid flushing the
page until some process first tries to execute from it.  If we
extended that to this scheme, when we made a segment executable, we
would have to find and flush all icache-dirty pages in the segment (up
to 65536 pages).  We wouldn't want to do that every time we made a
segment executable - it would need to be optimized (e.g. keep a count
per segment of icache-dirty pages in the segment).

Paul.