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SubjectRe: Allow swapping over nbd and more similar magic
On Sat, 18 Dec 1999, Pavel Machek wrote:

> This is (quite simple) patch to linux, which makes it possible to use
> things like nbd and crypted file for swapping. It was tested by
> several people and seems to work. It makes linux less picky about
> devices it can swap on...

It seems that your test people were just lucky,
kreclaimd as below should not do much that
kswapd doesn't do...

> --- clean/mm/vmscan.c Sat Dec 11 21:44:13 1999
> +++ linux/mm/vmscan.c Tue Dec 7 22:17:22 1999

> /*
> + * The background page discarder, started from the init process.
> + *
> + * This basically executes once a second, trickling out pages
> + * so that we have _some_ free memory available even if there
> + * is no other activity that frees anything up.
> + * We want this to be separate from kswapd, because we do not
> + * want to block, no matter what happens. As long as there are
> + * some clean pages backed it file (which is true under normal
> + * conditions, SCT tells me), we can swap over anything.

I guess this means that you want to do two things:
- swap out dirty pages when we have memory left
- swap out / free _clean_ pages when we are short on
memory

That is something completely different from the naive
tactic you're trying to use in the next code fragment.

> +int kreclaimd(void *unused)

[snip]

> + while (1) {
> + do {
> + int count = SWAP_CLUSTER_MAX;
> +
> + /* Normally, lets let kswapd do its work. If memory gets really low,
> + we'll want to clean up something to break the deadlock. */
> + if (nr_free_pages() >= freepages.low)
> + break;
> + while (do_try_to_free_pages(0) && count--) /* We don't want it to do any I/O */
> + ;
> +// printk( "kreclaimd: Freed %d pages\n", SWAP_CLUSTER_MAX - count );
> + } while (!tsk->need_resched);
}

Now what if:
- there already are a number of async swapouts
in progress (likely since kreclaimd only runs if
memory is tight)
- kreclaimd tries to swap out a few dirty pages
and runs into trouble

I guess you want to change the system a bit so that
one daemon only reclaims clean pages and the other
daemon has a condition where it cleans up dirty
pages without freeing memory on a periodic basis.

(ie. kswapd calling swap_out() but never shrink_mmap()
when free memory is high enough)

regards,

Rik
--
The Internet is not a network of computers. It is a network
of people. That is its real strength.


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