Re: BUG_ON() in pfn_swap_entry_to_page()

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 23, 2022 at 11:29:12AM +1100, Alistair Popple wrote:
> Matthew Wilcox <willy@infradead.org> writes:
> > It's not been reported before, but it seems obvious why it triggers.
> > I think it's ffa65753c431 "mm/migrate.c: rework migration_entry_wait()
> > to not take a pageref".
> 
> It's obvious the BUG_ON() is because the page isn't locked, but it's less
> obvious to me at least why we have a migration entry pointing to an unlocked
> page.
> 
> > There's a lot of inlining going on, so let's write this out:
> >
> > __handle_mm_fault
> >   handle_pte_fault
> >     do_swap_page():
> >
> >         entry = pte_to_swp_entry(vmf->orig_pte);
> >         if (unlikely(non_swap_entry(entry))) {
> >                 if (is_migration_entry(entry)) {
> >                         migration_entry_wait(vma->vm_mm, vmf->pmd,
> >                                              vmf->address);
> >
> > We don't (and can't) have the underlying page locked here.  We're just
> > waiting for it to finish migration.
> 
> Why can't the page be locked here? Waiting for migration to finish is equivalent
> to waiting for the page to be unlocked. __unmap_and_move() follows this rough
> sequence:
> 
> __unmap_and_move()
> 	if (!trylock_page(page)) {
> 		if (!force || mode == MIGRATE_ASYNC)
> 			goto out;
> 
> /*
> * It's not safe for direct compaction to call lock_page.
> * For example, during page readahead pages are added locked
> * to the LRU. Later, when the IO completes the pages are
> * marked uptodate and unlocked. However, the queueing
> * could be merging multiple pages for one bio (e.g.
> * mpage_readahead). If an allocation happens for the
> * second or third page, the process can end up locking
> * the same page twice and deadlocking. Rather than
> * trying to be clever about what pages can be locked,
> * avoid the use of lock_page for direct compaction
> * altogether.
>  */
> if (current->flags & PF_MEMALLOC)
>     goto out;
> 
>     lock_page(page);
> }
> 
> [...]
> 
> if (unlikely(!trylock_page(newpage)))
>     goto out_unlock;
> 
> [...]
> 
> } else if (page_mapped(page)) {
>     /* Establish migration ptes */
>     VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
>             page);
>     try_to_migrate(page, 0);
>     page_was_mapped = true;
> }
> 
> if (!page_mapped(page))
>     rc = move_to_new_page(newpage, page, mode);
> 
> if (page_was_mapped)
>     remove_migration_ptes(page,
>         rc == MIGRATEPAGE_SUCCESS ? newpage : page, false);
> 
> Both the source and destination pages (page and newpage respectively) are locked
> prior to installing migration entries in try_to_migrate() and unlocked after
> migration entries are removed. In order to remove a migration entry the PTL is
> required. Therefore while holding the PTL for a migration entry it should be
> impossible to observe it pointing to an unlocked page.

Hm, right.  We check the PTE initially, then take the PTL in
__migration_entry_wait() and then re-check that it's still a migration
entry.  I can't think of how we're seeing this ...