Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place

[Matthew Wilcox]

On Sun, Aug 08, 2021 at 10:55:30AM +0800, Baolin Wang wrote:
> Hi,
> 
> > On Fri, Aug 06, 2021 at 11:07:18AM +0800, Baolin Wang wrote:
> > > Hi Matthew,
> > > 
> > > > On Thu, Aug 05, 2021 at 11:05:56PM +0800, Baolin Wang wrote:
> > > > > We've got the expected count for anonymous page or file page by
> > > > > expected_page_refs() at the beginning of migrate_page_move_mapping(),
> > > > > thus we should move the page count validation a little forward to
> > > > > reduce duplicated code.
> > > > 
> > > > Please add an explanation to the changelog for why it's safe to pull
> > > > this out from under the i_pages lock.
> > > 
> > > Sure. In folio_migrate_mapping(), we are sure that the migration page was
> > > isolated from lru list and locked, so I think there are no race to get the
> > > page count without i_pages lock. Please correct me if I missed something
> > > else. Thanks.
> > 
> > Unless the page has been removed from i_pages, this isn't a correct
> > explanation.  Even if it has been removed from i_pages, unless an
> > RCU grace period has passed, another CPU may still be able to inc the
> > refcount on it (temporarily).  The same is true for the page tables,
> > by the way; if someone is using get_user_pages_fast(), they may still
> > be able to see the page.
> 
> I don't think this is an issue, cause now we've established a migration pte
> for this migration page under page lock. If the user want to get page by
> get_user_pages_fast(), it will wait for the page miggration finished by
> migration_entry_wait(). So I still think there is no need to check the
> migration page count under the i_pages lock.

I don't know whether the patch is correct or not, but you aren't nearly
paranoid enough.  Consider this sequence of events:

CPU 0:				CPU 1:
get_user_pages_fast()
lockless_pages_from_mm()
local_irq_save()
gup_pgd_range()
gup_p4d_range()
gup_pud_range()
gup_pmd_range()
gup_pte_range()
pte_t pte = ptep_get_lockless(ptep);
				migrate_vma_collect_pmd()
				ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl)
				ptep_get_and_clear(mm, addr, ptep);
page = pte_page(pte);
				set_pte_at(mm, addr, ptep, swp_pte);
				migrate_page_move_mapping()
head = try_grab_compound_head(page, 1, flags);

... now page's refcount is temporarily higher than it should be.  CPU 0
will notice the PTE is no longer the PTE that it used to be and drop
the reference, but in the meantime, CPU 1 can observe the higher refcount.

None of this has anything to do with the i_pages lock.  Holding it does
not protect from this race, but you need to know this kind of thing to
decide if changing how we test a page's refcount is safe or not.