Re: [Lhms-devel] [RFC 0/6] Swapless Page Migration V1: Overview

[Lee Schermerhorn <Lee.Schermerhorn@hp.com>]

On Mon, 2006-04-03 at 23:57 -0700, Christoph Lameter wrote:
> Swapless Page migration
> 
> Currently page migration is depending on the ability to assign swap entries
> to pages. This means that page migration will not work without swap although
> that swap space is never used.
> 
> This patchset removes that dependency by introducing a special type of
> swap entry that encodes a pfn number of the page being migrated. If that
> swap pte is encountered then do_swap_page() will simply wait for the page
> to become unlocked again (meaning page migration is complete) and then refetch
> the pte. The special type of swap entry is only in use while the page to be
> migrated is locked and therefore we can hopefully get away with just a few
> supporting functions.
> 
> To some extend this covers the same ground as Lee's and Marcelo's migration
> cache. However, I hope that this approach simplifies things without opening
> up any holes. Please check.
> 

Christoph:

Does this approach still allow "migrate-on-fault" for anon pages?
Especially, in the case where the migrating page has >1 pte referencing
it?  How will the fault handler find all of the pte's referencing the
old page?  Actually, I don't think we'd want to burden the task whose
fault caused the migration with finding and replacing and replacing all
pte's referecing the old page.  Using a real cache, this isn't a problem
because we replace the old page with a new one in the cache, and the
cache ptes reference the cache entry.  Tasks are free to fault in a real
pte for the new page at any time.  I'd hate to lose this capability.  I
believe that this is one of the reasons that Marcello used a real idr-
based cache for the migration cache.

Lee

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