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

Linus Torvalds (
Thu, 30 Jul 1998 20:05:36 -0700 (PDT)

On Thu, 30 Jul 1998, Alan Cox wrote:
> It would be better to fix DOSemu. Firstly about 2/3rds of the DOSemu uses
> seem to be fixable now we have mremap(). The others - aliasing emulations
> are less funny but could be messily done with sys5 shm.
> 2.0.x question - why did this work ok in 2.0 (did it in fact work ok in 2.0)

It has never worked correctly, and I think the thing is that the warning
is new to 2.1.x.

It _almost_ works. Problems crop up when you are low on memory, and a page
gets paged out - a page that used to be shared gets essentially
"duplicated" into two private pages by the page-out.

There used to be code that refused to page out pages with a count > 1, and
I can't remember offhand whether that test was in 2.0.x or not. I know
it's not in current kernels, the problem being exactly that with the test
you can create a major security issue by essentially locking down infinite
amounts of pages..

(I just checked, the test for page->count != 1 doesn't seem to be there in
2.0.x either, but I might have forgotten exactly where it used to be..)


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