Messages in this thread | | | Subject | Re: [PATCH 2/4] Add replace_page(), change the mapping of pte from one page into another | From | Lee Schermerhorn <> | Date | Wed, 12 Nov 2008 17:09:03 -0500 |
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On Wed, 2008-11-12 at 14:27 -0600, Christoph Lameter wrote: > On Wed, 12 Nov 2008, Andrea Arcangeli wrote: > > > On Tue, Nov 11, 2008 at 09:10:45PM -0600, Christoph Lameter wrote: > > > get_user_pages() cannot get to it since the pagetables have already been > > > modified. If get_user_pages runs then the fault handling will occur > > > which will block the thread until migration is complete. > > > > migrate.c does nothing for ptes pointing to swap entries and > > do_swap_page won't wait for them either. Assume follow_page in > > If a anonymous page is a swap page then it has a mapping. > migrate_page_move_mapping() will lock the radix tree and ensure that no > additional reference (like done by do_swap_page) is established during > migration.
So, it's Nick's reference freezing you asked about in response to my mail that prevents do_swap_page() from getting another reference on the page in the swap cache just after migrate_page_move_mapping() checks the ref count and replaces the slot with new swap pte. Radix tree lock just prevents other threads from modifying the slot, right? [Hmmm, looks like we need to update the reference to "write lock" in the comments on the 'deref_slot() and _replace_slot() definitions in radix-tree.h.]
Therefore, do_swap_page() will either get the old page and raise the ref before migration check, or it will [possibly loop in find_get_page() and then] get the new page.
Migration will bail out, for this pass anyway, in the former case. In the second case, do_swap_page() will wait on the new page lock until migration completes, deferring any direct IO.
Or am I still missing something?
> > > However it's not exactly the same bug as the one in fork, I was > > talking about before, it's also not o_direct specific. Still > > So far I have seen wild ideas not bugs.
Maybe not so wild, given the complexity of these interactions...
Later, Lee
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