Re: [PATCH 8/8] Mark page cache pages as __GFP_PAGECACHE instead of __GFP_MOVABLE

[Christoph Lameter <clameter@sgi.com>]

On Tue, 15 May 2007, Mel Gorman wrote:

> On Tue, 15 May 2007, Christoph Lameter wrote:
> 
> > On Tue, 15 May 2007, Mel Gorman wrote:
> > 
> > > Currently page cache pages are grouped with MOVABLE allocations. This
> > > appears
> > > to work well in practice as page cache pages are usually reclaimable via
> > > the LRU. However, this is not strictly correct as page cache pages can
> > > only
> > > be cleaned and discarded, not migrated. During readahead, pages may also
> > > exist on a pool for a period of time instead of on the LRU giving them a
> > > differnet lifecycle to ordinary movable pages.
> > 
> > Sorry but pagecache pages can be migrated.
> > 
> 
> Poor phrasing prehaps. I was under the impression that page migration was only
> concerned with pages mapped by process page tables for the move_pages() call.
> The statement above was also referring to pages read by readahead and normal
> file IO. I'm pretty sure they could be migrated without difficulty though once
> the source pages are identified. Either way, the separate grouping of page
> cache is probably not worthwhile for the moment.

So page cache = unmapped I/O pages? These can also be migrated. They still 
carry a refcount of the radix tree and page migration will have to update 
that pointer.

Page migration in its current form is indeed only used to move mapped 
pages but that is incidental to the current usage patterns. It is intended 
to be a generic page migration framework.

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