> > oops, right. The swap offset (the pfn of the swap-page) was encoded
> > as-is into the pte's higher 24 bits even in 2.2, this gives a limit of
> > 64GB swapspace on 32-bit platforms with 32-bit ptes. PAE mode raises
> > this limit to 16TB, as there are 32 bits worth of pfn encoded in the
> > upper 56 bits of the 64-bit pte.
>
> That makes some sense, thanks.
>
> How does the swap cache fit into this picture? Is that obsolete
> (or absent) under 2.3 now?
it's very much intact. The swap cache is embedded in the pagecache and
serves as a fast-lookup for recently swapped out pages. It's a 'backside'
cache to the swap device itself. A swapped out page is only truly freed
once it's evicted from the swap cache as well. The swap cache enables us
to do more aggressive and asynchronous swap-out - if we got it wrong (and
the page is faulted in before it's completely freed) we only take a minor
page fault to establish the mapping again.
-- mingo
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