Re: [PATCH 2.6.12-rc3 4/8] mm: manual page migration-rc2 -- add-sys_migrate_pages-rc2.patch

[Ray Bryant <raybry@engr.sgi.com>]

Hirokazu Takahashi wrote:


> 
> I just thought of the page, belonging to some file which is
> mmap()ed to the target process to be migrated. The page may
> not be accessed and the associated PTE isn't set yet.
> if vma->vm_file->f_mapping equals page_mapping(page), the page
> should be migrated. 
> 
> Pages in the swap-cache have the same problem since the related
> PTEs may be clean.
> 
> But these cases may be rare and your approach seems to be good
> enough in most cases.
> 
> 

Well, what could be done would be the following, I suppose:

If follow_page() returns NULL and the vma maps a file, we could
lookup the page in the radix tree, and if we find it, and if it
is on a node that we are migrating from, we could add the page
to the set of pages to be migrated.

The disadvantage of this is that we could do a LOT of radix
tree lookups and find relatively few pages.  (Our approach of
releasing free page cache pages first makes such pages just
"go away".  But we don't have "release free page cache pages"
in the mainline yet.  :-( )

Similarly, if we modified follow_page() (e. g. follow_page_ex())
to return the pte, check to see if it is a swap pte (!pte_none()
&& !pte_file()), if so then use pte_to_swap_entry() to get
the swap entry, and then use that to look up the page in the
swapper space radix tree.  Then handle it as above (hmmm...
will your migration code handle a page in the swap cache?)

Once again, this could lead to lots of lookups.  This is
especially a concern for a large multithreaded app, since the
address spaces are the same for each process id, hence we look
up the same info over and over in each page table scan.

-- 
Best Regards,
Ray
-----------------------------------------------
                   Ray Bryant
512-453-9679 (work)         512-507-7807 (cell)
raybry@sgi.com             raybry@austin.rr.com
The box said: "Requires Windows 98 or better",
            so I installed Linux.
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