Re: [PATCH] avoid unnecessary cache flushes

Paul Mackerras
2001-09-04T02:00:52Z

Richard Henderson writes:

> You can get a missed flush from
>
> bit == 0
> flush cache
>
> modify page
> bit = 0
>
> bit = 1
>
> unless this is protected from some outer lock of which
> I am not aware.

The page is question is one which is mapped (is being mapped) into a
process's address space with execute permission. If the page is part
of the page cache for a file, then I would have thought that attempts
to write to the page would return an ETXTBSY error. If the page is a
private page (private COW or anonymous page) then I don't see where
the kernel would be modifying the page. If it is already mapped into
another process's address space with a shared writable mapping, and
that process is writing to the page, then it is up to the user-level
stuff to do the necessary cache-flushing.

So I think it's OK at the moment, but I agree it does look like a race
waiting to happen.

For alpha, the thing that my patch does that might hurt is the change
from flush_icache_page to flush_icache_range in kernel/ptrace.c. Any
comment on that?

Paul.