Messages in this thread | | | Subject | Re: 2.6.17-rc5-mm1 | From | Martin Schwidefsky <> | Date | Tue, 06 Jun 2006 19:48:16 +0200 |
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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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