Re: [PATCH 2.6.17-rc1-mm1 2/6] Migrate-on-fault - check for misplaced page

[Christoph Lameter <clameter@sgi.com>]

On Tue, 11 Apr 2006, Lee Schermerhorn wrote:

> > The misplaced page function should not consider the vma policy if the page 
> > is mapped because the VM does not handle vma policies for file 
> > mapped pages yet. This version may be checking for a policy that would
> > not be applied to the page for regular allocations.
> 
> When you say "mapped" here, you mean a mmap()ed file?  As opposed to
> "mapped by a pte" such that page_mapcount(page) != 0, right?  Because if
> the mapcount() isn't zero, we won't even look for misplaced pages.  And,
> with the V0.2 series, I'm only checking for misplaced pages with
> mapcount == 0 in the anon page fault path.  If necessary, I can skip
> pages in VMAs that have non-NULL vm_file.  Do we get these in the anon
> fault path?

You would need to skip evaluating the vma policy for file backed pages
for the misplaced page check.

> > You need to use the task policy instead of the vma policy if the page is 
> > file backed because vma policies do not apply in that case.
> 
> OK, but again, I haven't hooked up migrate-on-fault for file backed
> pages yet.  Here, you're saying that if I DID hook it up before fixing
> how file back pages are handled, then to be consistent with current
> behavior, I should use task policy for file back pages?

If this applied only to anonymous pages then its okay.

> How about shmem backed pages?

Those have a valid policy even when they are unmapped.

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