On Tue, 2006-06-06 at 10:39 -0700, Christoph Lameter wrote:
> Ok. So we have a mystical expression here that needs to be explained:
>
> swp_type(pte_to_swp_entry(swp_entry_to_pte(swp_entry(~0UL,0))))
>
> swp_entry(~0L,0) ->
> swp_entry(0xffffffffffffffff, 0) ->
> 0xfffffffffffffff << SWP_TYPE_SHIFT ->
> 0x1f
>
> Ok we got the highest swap type and this encodes MAX_SWAPFILES_SHIFT and
> therefore limits the number of swap
>
> But now we convert this to a arch specific pte. The hope is that the arch
> will mask off any unsupported bits? This is done by s390?
Yes.
> Then we convert the result back into a swap entry and take its type which
> will hopefully give us the maximum number of swap devices supported by the
> arch?
Yes.
> Would it not be better for an arch to explicitly tell us how many swap
> devices are supported? Then we could make the swap_info array shorter and
> get rid of this cryptic test.
Do you want to calculate the correct number for all architectures? Have
fun trying to decrypt the different pte formats.
> This test also creates a problem for the migration entries: It is really
> not clear until runtime how many swap devices are supported so we cannot
> take the last 2 for page migration. In order for page migration to work
> all NUMA arches that do not support 32 swap devices need to define
> CONFIG_MIGRATION=n. Seems that this is only s390?
s390 is not a NUMA system, but it does support 32 swap devices.
--
blue skies,
Martin.
Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH
"Reality continues to ruin my life." - Calvin.
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