Christoph Lameter <[email protected]> wrote:
>
> On Thu, 13 Apr 2006, Andrew Morton wrote:
>
> > So we falsely return VM_FAULT_MINOR and let userspace retake the pagefault,
> > thus implementing a form of polling, yes? If so, there is no "something
> > else" which this process can do.
>
> Right.
>
> > Pages are locked during migration. The faulting process will sleep in
> > lock_page() until migration is complete. Except we've gone and diddled
> > with the swap pte so do_swap_page() can no longer locate the page which
> > needs to be locked.
>
> Oh. The page is enconded in the migration pte.
>
> > Doing a busy-wait seems a bit lame. Perhaps it would be better to go to
> > sleep on some global queue, poke that queue each time a page migration
> > completes?
>
> If we rely on the migrating thread to hold the page count while the
> page is locked then we could do what the patch below does. But then we
> may race with the freeing of the old page after migration is finished.
Yeah, that's unpleasant.
> If we would add the
> increment of the page count back then we are on the safe side but have
> the problem that we may increment the page count before the migrating
> thread gets to the final check. Then the migration check would fail
> and we would retry.
>
>
> Index: linux-2.6.17-rc1-mm2/mm/memory.c
> ===================================================================
> --- linux-2.6.17-rc1-mm2.orig/mm/memory.c 2006-04-13 17:32:36.000000000 -0700
> +++ linux-2.6.17-rc1-mm2/mm/memory.c 2006-04-13 18:26:49.000000000 -0700
> @@ -1881,11 +1881,11 @@ static int do_swap_page(struct mm_struct
> entry = pte_to_swp_entry(orig_pte);
>
> if (is_migration_entry(entry)) {
> - /*
> - * We cannot access the page because of ongoing page
> - * migration. See if we can do something else.
> - */
> - yield();
> + page = migration_entry_to_page(entry);
> + lock_page(page);
> + entry = pte_to_swp_entry(*page_table);
> + BUG_ON(is_migration_entry(entry));
> + unlock_page(page);
> goto out;
> }
Is this page still lookable-uppable in swapcache? If so, that's the way to
get the refcount on it.
We don't _have_ to use the page lock of course. A simple
wait_event(some_wq, !is_migration_entry(entry));
would suffice.
But what prevents this swp_entry_t from becoming an is_migration_entry
swp_pte_t two nanoseconds after we've passed this check?
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