Re: [PATCH mm] swapless page migration: fix swapops.h:97 BUG

[Christoph Lameter <clameter@sgi.com>]

On Wed, 24 May 2006, Hugh Dickins wrote:

> Such an orphan may be left behind if an earlier migration raced with fork:
> copy_one_pte can duplicate a migration entry from parent to child, after
> remove_anon_migration_ptes has checked the child vma, but before it has
> removed it from the parent vma.  (If the process were later to fault on
> this orphaned entry, it would hit the same BUG from migration_entry_wait.)

Right the pte lock does not help us there. sigh.

> This could be fixed by locking anon_vma in copy_one_pte, but we'd rather
> not.  There's no such problem with file pages, because vma_prio_tree_add
> adds child vma after parent vma, and the page table locking at each end
> is enough to serialize.  Follow that example with anon_vma: add new vmas
> to the tail instead of the head.

Thats is very subtle. I hope there are no other areas where the child vma 
has to be processed before the parent vma? An alternativbe would be to 
make remove_anon_migration_ptes walk the list in reverse and add a big 
comment.

Thank you.