32GB limit per swap device?

[Alan Cox]

On Thu, Aug 18, 2011 at 3:16 AM, Alexander V. Chernikov <melifaro at ipfw.ru>wrote:

> On 10.08.2011 19:16, perryh at pluto.rain.com wrote:
>
>> Chuck Swiger<cswiger at mac.com>  wrote:
>>
>>  On Aug 9, 2011, at 7:26 AM, Daniel Kalchev wrote:
>>>
>>>> I am trying to set up 64GB partitions for swap for a system that
>>>> has 64GB of RAM (with the idea to dump kernel core etc). But, on
>>>> 8-stable as of today I get:
>>>>
>>>> WARNING: reducing size to maximum of 67108864 blocks per swap unit
>>>>
>>>> Is there workaround for this limitation?
>>>>
>>>
> Another interesting question:
>
> swap pager operates in page blocks (PAGE_SIZE=4k on common arch).
>
> Block device size in passed to swaponsomething() in number of _disk_ blocks
>  (e.g. in DEV_BSIZE=512). After that, kernel b-lists (on top of which swap
> pager is build) maximum objects check is enforced.
>
> The (possible) problem is that real object count we will operate on is not
> the value passed to swaponsomething() since it is calculated in wrong units.
>
> we should check b-list limit on (X * DEV_BSIZE512 / PAGE_SIZE) value which
> is rough (X / 8) so we should be able to address 32*8=256G.
>
> The code should look like this:
>
> Index: vm/swap_pager.c
> ==============================**==============================**=======
> --- vm/swap_pager.c     (revision 223877)
> +++ vm/swap_pager.c     (working copy)
> @@ -2129,6 +2129,15 @@ swaponsomething(struct vnode *vp, void *id, u_long
>        u_long mblocks;
>
>        /*
> +        * nblks is in DEV_BSIZE'd chunks, convert to PAGE_SIZE'd chunks.
> +        * First chop nblks off to page-align it, then convert.
> +        *
> +        * sw->sw_nblks is in page-sized chunks now too.
> +        */
> +       nblks &= ~(ctodb(1) - 1);
> +       nblks = dbtoc(nblks);
> +
> +       /*
>
>         * If we go beyond this, we get overflows in the radix
>         * tree bitmap code.
>         */
> @@ -2138,14 +2147,6 @@ swaponsomething(struct vnode *vp, void *id, u_long
>                        mblocks);
>                nblks = mblocks;
>        }
> -       /*
> -        * nblks is in DEV_BSIZE'd chunks, convert to PAGE_SIZE'd chunks.
> -        * First chop nblks off to page-align it, then convert.
> -        *
> -        * sw->sw_nblks is in page-sized chunks now too.
> -        */
> -       nblks &= ~(ctodb(1) - 1);
> -       nblks = dbtoc(nblks);
>
>        sp = malloc(sizeof *sp, M_VMPGDATA, M_WAITOK | M_ZERO);
>        sp->sw_vp = vp;
>
>
> (move pages recalculation before b-list check)
>
>
> Can someone comment on this?
>
>
I believe that you are correct.  Have you tried testing this change on a
large swap device?

Alan