Re: writable swap cache explained (it's weird)

Benjamin C.R. LaHaise (
Thu, 30 Jul 1998 15:20:20 -0400 =?ISO-8859-1?Q?(`=EB=BA=FF=FF=FF=FFp?=)

On Wed, 29 Jul 1998, Bill Hawes wrote:

> To fix this I think we need to mark the whole mess as unswappable. It
> won't work to just test for a writable pte to a shared page -- in this
> case one of the sharings is only readable. So if the readable one gets
> swapped out first, the remaining mappings would still be a problem.
> Anyone have any ideas for the best way to detect and handle this case?

There are two options:

a) disallow MAP_SHARED mappings of anonymous memory from

b) implement shared anon mappings

(a) sounds like the Obvious Thing To Do in the mmap method for /proc, but
will break xdos. Wtf were they thinking in writing that insane code?
Hmmm, this bug probably applies to 2.0 too.... in a much more subtle

As for (b), I'll try to present code by Saturday, as it is a nice feature
to add to our cap. =) (No, it's not going to be anything like the awful
shm code.)


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