Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

[Peter Xu <peterx@redhat.com>]

On Thu, Aug 25, 2022 at 10:42:41AM +1000, Alistair Popple wrote:
> 
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Wed, Aug 24, 2022 at 04:25:44PM -0400, Peter Xu wrote:
> >> On Wed, Aug 24, 2022 at 11:56:25AM +1000, Alistair Popple wrote:
> >> > >> Still I don't know whether there'll be any side effect of having stall tlbs
> >> > >> in !present ptes because I'm not familiar enough with the private dev swap
> >> > >> migration code.  But I think having them will be safe, even if redundant.
> >> >
> >> > What side-effect were you thinking of? I don't see any issue with not
> >> > TLB flushing stale device-private TLBs prior to the migration because
> >> > they're not accessible anyway and shouldn't be in any TLB.
> >>
> >> Sorry to be misleading, I never meant we must add them.  As I said it's
> >> just that I don't know the code well so I don't know whether it's safe to
> >> not have it.
> >>
> >> IIUC it's about whether having stall system-ram stall tlb in other
> >> processor would matter or not here.  E.g. some none pte that this code
> >> collected (boosted both "cpages" and "npages" for a none pte) could have
> >> stall tlb in other cores that makes the page writable there.
> >
> > For this one, let me give a more detailed example.
> 
> Thanks, I would have been completely lost about what you were talking
> about without this :-)
> 
> > It's about whether below could happen:
> >
> >        thread 1                thread 2                 thread 3
> >        --------                --------                 --------
> >                           write to page P (data=P1)
> >                             (cached TLB writable)
> >   zap_pte_range()
> >     pgtable lock
> >     clear pte for page P
> >     pgtable unlock
> >     ...
> >                                                      migrate_vma_collect
> >                                                        pte none, npages++, cpages++
> >                                                        allocate device page
> >                                                        copy data (with P1)
> >                                                        map pte as device swap
> >                           write to page P again
> >                           (data updated from P1->P2)
> >   flush tlb
> >
> > Then at last from processor side P should have data P2 but actually from
> > device memory it's P1. Data corrupt.
> 
> In the above scenario migrate_vma_collect_pmd() will observe pte_none.
> This will mark the src_pfn[] array as needing a new zero page which will
> be installed by migrate_vma_pages()->migrate_vma_insert_page().
> 
> So there is no data to be copied hence there can't be any data
> corruption. Remember these are private anonymous pages, so any
> zap_pte_range() indicates the data is no longer needed (eg.
> MADV_DONTNEED).

My bad to have provided an example but invalid. :)

So if the trylock in the function is the only way to migrate this page,
then I agree stall tlb is fine.

> 
> >>
> >> When I said I'm not familiar with the code, it's majorly about one thing I
> >> never figured out myself, in that migrate_vma_collect_pmd() has this
> >> optimization to trylock on the page, collect if it succeeded:
> >>
> >>   /*
> >>    * Optimize for the common case where page is only mapped once
> >>    * in one process. If we can lock the page, then we can safely
> >>    * set up a special migration page table entry now.
> >>    */
> >>    if (trylock_page(page)) {
> >>           ...
> >>    } else {
> >>           put_page(page);
> >>           mpfn = 0;
> >>    }
> >>
> >> But it's kind of against a pure "optimization" in that if trylock failed,
> >> we'll clear the mpfn so the src[i] will be zero at last.  Then will we
> >> directly give up on this page, or will we try to lock_page() again
> >> somewhere?
> 
> That comment is out dated. We used to try locking the page again but
> that was removed by ab09243aa95a ("mm/migrate.c: remove
> MIGRATE_PFN_LOCKED"). See
> https://lkml.kernel.org/r/20211025041608.289017-1-apopple@nvidia.com
> 
> Will post a clean-up for it.

That'll help, thanks.

-- 
Peter Xu