Re: BUG_ON() in pfn_swap_entry_to_page()

Matthew Wilcox
2022-03-24T03:24:18Z

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 ...